The 48-year-old driver of the van was detained by members of the public and then arrested by police.
No other suspects in the incident have been identified, and the vehicle used in the attack was rented from a van hire company based in Wales.
"Everybody was shouting: 'A van's hit people, a van's hit people, '" she said. Police said earlier that one person was arrested.
She was due to hold an emergency ministerial meeting on Monday.
Under pressure to soften Brexit, May meets Northern Irish 'kingmakers'
The opening was set to take place on Monday, but was delayed amid negotiations. May held talks with the Democratic Unionist Party (DUP) on Tuesday.
Kacimi said when he did so, the 48-year-old driver said to him: "I've done my bit".
London police, already stretched by a series of recent tragedies including a high-rise fire in which 79 people are presumed dead and a terror attack near London Bridge that killed seven people, said they are putting more officers on the street to reassure the public.
ONE man has died and eight people have been taken to hospital after a van was driven into pedestrians near a north London mosque. He added that the driver of the van was arrested on suspicion of murder.
London Mayor Sadiq Khan called the incident a "horrific terror attack".
Manchester was also hit on May 22 when a suicide bomber killed 22 people at an Ariana Grande concert.
Police are now working to identify the dead victim of the attack outside the mosque, although witnesses described him as being an older man.
The driver, who witnesses say screamed "I'm gonna kill all Muslims" after ploughing into worshippers, was pinned to the ground before police arrived.
US Service Members Wounded In 'Insider Attack' In Afghanistan
For the second time in a week, a man in an Afghan army uniform opened fire on USA service members at a base in Afghanistan . In March another insider attack killed three US soldiers. "We are aware of an incident", the office said in a statement.
"When I got him on the ground, I said, 'Why are you doing this?' He said, 'I want to kill more Muslims'".
The incident comes at a time when emotions are high in the United Kingdom, in the wake of a series of deadly terrorist attacks.
- Jeremy Corbyn, the leader of Britain's opposition Labour Party tweeted "I'm totally shocked at the incident at Finsbury Park tonight". Officials deployed armed officers at the scene, and several hours later, counter-terrorism police were brought in and are reportedly leading the investigation. I've been in touch with the mosques, police and Islington council regarding the incident.
The attacker struck as the area was busy with worshippers attending Ramadan night prayers at Finsbury Park mosque.
Photos show a number of police cars are on the scene and the area cordoned off to civilians.
Terrorists Khuram Butt, Rachid Redouane and Youssef Zaghba hired a van and ploughed into pedestrians on London Bridge before going on a knife rampage attacking people enjoying meals and drinks in Borough Market, killing eight people and leaving 21 in a critical condition.
Opened in 1994, Finsbury Park Mosque is an unassuming five-story redbrick building in residential north London, close to Arsenal Football Club's Emirates Stadium.
Donald Trump to restrict Cuba travel and trade in latest Obama rollback
That means those looking to participate in people-to-people educational exchanges will, again, have to do so with a group. However, the policy changes appear to allow Marriott, Starwood's parent company, to continue operating the hotel.