A crowdfunding campaign started by a Muslim man who witnessed the Westminster attack has raised more than £17,000 in just a day for victims of the atrocity and their families.

The Muslims United for London page passed the £3,000 mark within an hour of going live and surpassed £10,000 over the next 15 hours as hundreds of people rushed to donate.

Muddassar Ahmed said he started the fund after witnessing the attack, having entered Portcullis House just 10 minutes before terror struck and being barricaded inside for four hours.

“I was shocked to see the injuries and loss of life outside my window,” he wrote.

“I reflected on what it means to be a born-and-bred Londoner and found myself proud of how security and medical services responded, how ordinary passers-by offered first aid, and what our Parliament means to me, an institution that is the oldest of its kind in the world and how, regardless of our critiques of government policies or political parties, remains an institution that reflects how the will of the people can be expressed with civility and dignity.”

He has now raised the fund’s target to £20,000 because of the unexpected “high demand and heartwarming response”.

Candlelit vigil for Westminster attack victims held in Trafalgar Square

Donations continued to pour in on Friday morning after police announced a fourth victim of the attack had died.

A 75-year-old man suffered severe injuries and was receiving treatment in hospital before life support was withdrawn.

He was the third victim to die from injuries sustained as Khalid Masood ploughed his car into pedestrians on Westminster Bridge, killing mother-of-two Aysha Frade and American tourist Kurt Cochran, who was in London with his wife to celebrate their 25th anniversary.

Masood, a 52-year-old Muslim convert from Kent, then ploughed the vehicle into a fence outside the Houses of Parliament before running to an entrance and fatally stabbing PC Keith Palmer, before he was shot dead.

Isis hailed him as a “soldier of the Islamic State” in a claim of responsibility issued the following day, although the extent of any involvement by the terrorist group remains unclear.

Theresa May told MPs packed into the House of Commons that he had been the subject of a historical MI5 investigation over suspected violent extremism but was not charged with terror offences.

Born Adrian Elms and using a series of aliases through his life, Masood had been jailed repeatedly for violent crime dating back to the 1980s, including grievous bodily harm, possession of offensive weapons and assault.

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

A minute’s silence was held for his victims as faith leaders, the Home Secretary and Mayor of London joined thousands of Londoners at a memorial held in Trafalgar Square on Thursday evening.

Among the attendees was the head of the Muslim Council of Britain, Harun Khan, who condemned the “cowardly and depraved” terror attack.

“There is no justification for this act whatsoever,” he added.

“The best response to this outrage is to make sure we come together in solidarity and not allow the terrorists to divide us.”

Sadiq Khan urged Muslims to be “vigilant” against the dangers of Islamist ideology and hate preachers.

He urged people to ensure young people know “true Islam” from online propaganda to help them combat the risk of grooming and radicalistation.

“Terrorists want to attack London is because they hate the fact that we don’t just tolerate each other – whether you’re a Christian, Muslim, Jew, Hindu, Sikh, Buddhist, member of an organised faith or not, we respect, embrace and celebrate each other and that’s going to carry on,” Mr Khan said.