Mr Hildebrand had led the Swiss National Bank for two years
The chairman of the Swiss National Bank (SNB), Philipp Hildebrand, has resigned with immediate effect.
Mr Hildebrand has faced allegations of wrongdoing after reports he made one large trade shortly before the central bank capped its exchange rate.
It was also confirmed that his wife Kashya bought $504,000 (£323,024) in August, three weeks before the bank intervened. Both deny wrongdoing.
Mr Hildebrand will make a statement shortly.
The SNB said last week that Mr Hildebrand's wife had broken no rules.
Mr Hildebrand himself spoke about the issue for the first time on Thursday, denying any legal breaches, but admitting that some of the transactions "could bring my integrity into doubt". He also said at the time that he would not resign.
Tony Nyman, an analyst at Informa Global Markets, said Mr Hildebrand's position was "almost untenable and so it has proved".
"The Swiss franc has actually gained on the news possibly due to hopes of increased integrity ahead, but also market positioning too," he said.
"Once the news gets digested, we do not expect a lasting impact on the franc from the news, however."
Tear gas and the wail of ambulance sirens are a constant presence in Tahrir Square
Unrest is continuing in Cairo as protesters step up their demand for Egypt's military rulers to resign.
Street battles with riot police have been heaviest around the fortified interior ministry located on a side street off Tahrir Square.
Gunfire was reported late on Wednesday but the interior minister said security forces were only firing tear gas.
The protesters have rejected a pledge by the ruling military council to speed up transition to a civilian government.
The BBC's Jeremy Bowen in Cairo says the violence threatens to overshadow next week's parliamentary elections.
He says public opinion on the protests is divided. Some Egyptians want elections to go ahead unhindered while others believe the military must be swept from power first.
The clashes, now entering their sixth day, are the longest outbreak of violence since the 18-day uprising that toppled former President Hosni Mubarak in February.
Since then a military council has been tasked with guiding the country's transition to democracy. The latest protests have been triggered by suspicions that the military intends to hang on to power.
On Tuesday Field Marshal Mohamed Hussein Tantawi, head of the Supreme Council of the Armed Forces, sought to defuse the situation by promising presidential elections by the end of June, six months sooner than planned. The military-appointed civilian cabinet also tendered its resignation.
"They [the military] were with Mubarak from the start," said Fatihia Abdul Ezz, 60, who had come to the square to protest.
"I came when I saw our sons being killed."
Steel buckshot
Groups of stone-throwing demonstrators have been locked in pitched battles in the streets between Tahrir Square and the interior ministry for several days.
Riot police blocking streets leading to the ministry have driven back the protesters with volleys of tear gas and rubber-covered steel buckshot.
Protesters have spoken of gunshots and injuries or deaths from live bullets but Interior Minister Mansour el-Essawy said security forces were only firing tear gas.
On state TV he said unidentified people were shooting from rooftops near Tahrir Square, Reuters news agency said.
At one point on Wednesday a truce was apparently brokered by religious leaders, but soon broke down and the clashes continued into the night.
There were also clashes in Egypt's second city, Alexandria, and in Ismailia, on the Suez Canal.
Television pictures from Ismailia showed armoured vehicles patrolling streets as security forces tried to disperse protesters with volleys of tear gas.
In Alexandria protests have been smaller than in Cairo, but one protester said clashes were continuing early on Thursday outside the security headquarters.
One protester, Mahinour, told the BBC the number of injured people there was increasing.
"Most of them are suffocating because of gas. This time they are not using tear gas, it's more nerve gas than tear gas. And as well there are some people injured by rubber bullets," she said.
Despite the clashes, the BBC's Wyre Davies in Alexandria says the appetite for conflict is lessening and people want their country to move on.
'Excessive force'
The main opposition group, the Muslim Brotherhood, is not supporting the protests and expects to do well in the parliamentary elections which start next week and will be staggered over several months.
Earlier on Wednesday, UN human rights chief Navi Pillay condemned the "clearly excessive use of force" by Egypt's security forces during the clashes.
She called for an independent inquiry into deaths since the weekend.
The health ministry said on Wednesday that 35 people had died in clashes since Saturday - all but four in Cairo. Hundreds more have been injured.
State news agency Mena reported that one person had been shot dead in the north-western city of Mersa Matruh as demonstrators tried to storm a police station.
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The BBC's Yolande Knell said protesters were angry about the power held by the military
At least one person has been killed and more than 600 wounded in fierce clashes between protesters and security forces in central Cairo.
The clashes came as police moved to prevent a long-term sit-in following a huge demonstration against the military leadership on Friday.
Some protesters lobbed rocks and a police vehicle was set on fire.
The latest violence comes just over a week before parliamentary elections are scheduled to begin.
Protesters - mostly Islamists and young activists - have been holding demonstrations against a draft constitution that they say would allow the military to retain too much power after a new civilian government is elected.
Witnesses said there were also demonstrations in Suez and Alexandria.
Rubber bullets
Correspondents say many Egyptians are frustrated at what they see as a reluctance by the ruling military council - who took power after the fall of former President Hosni Mubarak in February - to carry out meaningful reforms.
The clashes began as police moved into the square to break up a sit-in
Saturday's violence began when police moved to dismantle tents erected by demonstrators in Cairo's Tahrir Square who had camped out for the night.
The move to clear the square prompted thousands of protesters to return, and clashes erupted. Police fired rubber bullets as cars were set on fire, witnesses said.
The numbers of protesters swelled, reportedly after a call went out on social media for people to join the demonstration following the police assault.
Prime Minister Essam Sharaf called on the protesters to clear the square.
"What is happening in Tahrir is very dangerous and threatens the course of the nation and the revolution," a statement from the cabinet said.
Black smoke rose over the square after protesters swarmed over an armoured police van, turned it over and set it alight, reports said.
The state-run Mena news agency says more than 670 people were injured.
One of those wounded later died in hospital, the agency adds.
AFP quoted a security official as saying a number of arrests were made.
'Harsh beatings'
One of the demonstrators, Ali Abdel Aziz, said security forces beat up protesters to break up the sit-in on Saturday.
"They beat us harshly, they didn't care for either men or women," the 32-year-old accountancy professor told AFP news agency.
"The interior ministry must take responsibility. We have one demand, the military council must go," he said.
One prominent protester, Malek Mostafa, lost an eye after being hit by a rubber bullet, said Ghada Shahbender, a member of the Egyptian Organisation for Human Rights.
"It is a crime," she told AP news agency. "They [police] were shooting rubber bullets directly at the heads... I heard an officer ordering his soldiers to aim for the head."
The interior ministry says a number of policemen were among the wounded.
In Alexandria, protester Ahmed Abdel-Qader told the Associated Press activists had thrown stones at the main security headquarters.
"We only managed to bring down the head of the regime [Hosni Mubarak]. The rest of the tree is still standing," he said.
Friday's demonstration, which saw tens of thousands fill Tahrir Square, was one of the largest for months.
Parliamentary elections are due to begin on 28 November and take three months.
Earlier this month, Egypt's military rulers produced a draft document setting out principles for a new constitution.
Under those guidelines, the military would be exempted from civilian overnight, as would its budget.
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Mr Hildebrand had led the Swiss National Bank for two years
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