A crowdfunding campaign by a British Muslim who witnessed an attack in London that killed four people has almost reached a target of $37,000, which will be donated to the victims' families.

By the time of publishing, more than 1,000 people had donated $33,500 towards the fund.

On Wednesday, Muddassar Ahmed was barricaded in a nearby office building in Westminster at the time of the car-and-knife attack, which also wounded dozens.

"I was there and I was shocked," he told Al Jazeera. "Because I saw what happened, I felt that had to do something for the victims ... I wanted to raise money quickly so I launched the campaign late that night."

"It was primarily to help families of the victims ... it was something that could be done to offer support and empathise with the families."

Ahmed then called his friends to help with fundraising.

Two British Muslim MPs - Naz Shah and Yasmin Qureshi - have since voiced their support for the Muslims United for London initiative.

'Being proactive is part of the solution'

On the crowdfunding page, Ahmed wrote that he was shocked to witness "the injuries and loss of life outside my window".

"I reflected on what it means to be a born-and-bred Londoner and found myself proud of how security and medical services responded," he said.

The suspect behind the attack, who was killed after he stabbed a policeman to death, was named as 52-year-old Khalid Masood. Reports said he was a Muslim convert.

Some 40 people were injured in the attack, 29 of whom were being treated in hospital, according to police. Seven were still in critical condition.

"Rather than just condemning the attacks, being proactive and actually doing something is part of the solution," said Ahmed.

While acknowledging that the campaign might help to show Muslims in a positive light, that was not his "primary goal".

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Some of Britain's Muslim community leaders remain concerned about a possible backlash in the aftermath of the attack.

Following similar violence across Europe in recent years, Muslims have suffered collective punishment as some equate Islam with "terrorism".

In London, police have advised several mosques to increase security.

"We're in a very difficult position, the Muslim community," Mohammed Kozbar, chairman of the Finsbury Park mosque, told Al Jazeera.

"We hope that there's no reaction from some far-right extremists who may use this incident to spread fear and hate and racism among our society."

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Meanwhile, religious leaders from different faiths have gathered near the site of the attack to call for unity.

"To Christians, to Anglicans, who sense a great emotion of anger, that is appropriate when the innocent are killed," said Justin Welby, the archbishop of Canterbury. "It is not appropriate to be angry at a whole category of people, but simply at one person."

Woman in hijab abused on social media

Following Wednesday's attack, a photograph picturing a Muslim woman walking at the scene went viral, with many questioning her willingness to aid the wounded.

Thousands of people on Twitter shared the image, which drew a barrage of racist messages.

The Muslim woman, who wishes to remain anonymous, later released a statement through Tell MAMA, a UK-based organisation that keeps track of Islamophobic attacks.

"My thoughts at that moment were one of sadness, fear, and concern," the woman said. "What the image does not show is that I had talked to other witnesses to try and find out what was happening, to see if I could be of any help, even though enough people were at the scene tending to the victims."

She added that she was "devastated" by the attack, and at the shock of finding her picture plastered all over social media "by those who could not look beyond my attire, who draw conclusions based on hate and xenophobia".

Additional reporting by Zineb Abdessadok, Neave Barker, Barnaby Phillips.