London, UK: Eight officers fired an ‘unprecedented’ 50 rounds at the three attackers, according to Metropolitan Police Assistant Commissioner Mark Rowley, adding that a member of the public also suffered a gunshot wound.

Detectives were still investigating whether the assailants acted alone, but Rowley said he was ‘increasingly confident that this attack was conducted by three individuals’.

The 12 arrests were made in the ethnically diverse east London suburb of Barking, with Sky News reporting that a property raided by police belonged to one of the killers.

May said the attack was driven by the same ‘evil ideology of Islamist extremism’ behind last week’s Manchester suicide bombing that left 22 people dead, and the Westminster attack in March, which killed five.

‘The recent attacks are not connected but we believe we are experiencing a new trend in the threat we face,’ she said after chairing a meeting of the government’s emergency Cobra committee.

She warned that perpetrators are inspired to attack ‘by copying one another’.

The assailants wore fake suicide vests in a bid to increase the sense of panic as they ran people over on London Bridge before lunging seemingly at random at the crowds gathered around Borough Market, which is full of restaurants and bars.

Gerard Vowls, 47, said he saw a woman repeatedly stabbed, and threw chairs, glasses and bottles at the attackers in a bid to stop them.

‘They kept coming to try to stab me... they were stabbing everyone. Evil, evil people,’ he told The Guardian newspaper.

Another witness called Eric told the BBC he had seen three men get out of the van and thought they were going to help tend those who had been run over.

Instead they ‘started kicking them, punching them and took out knives. It was a rampage really,’ he said, adding that he heard a shout of: ‘This is for Allah’.

Saturday night's rampage at a popular nightlife hub around London Bridge by three men wearing fake suicide vests was the third deadly terror attack in Britain in less than three months and came only days before a general election.

‘A detachment of fighters from Islamic State carried out London attacks yesterday,’ said the Aamaq news agency, which is affiliated with the Islamic  State group.

National campaigning for Thursday's vote was suspended for the day out of respect for the victims, who included 48 people treated in hospital for injuries.

Of those, 21 are still in a critical condition.

One Canadian national and one Frenchman were among the fatalities and seven French citizens were among the injured.

No details have been released about the perpetrators, who were shot dead within minutes by police.