British Prime Minister Theresa May has called for a stronger response to Islamist extremism after three attackers drove a van into pedestrians on London Bridge and stabbed others nearby, killing seven people and injuring 48.
Citing NHS England, the Press Association reported that 21 of those injured are in a critical condition.
London's Metropolitan Police arrested 12 people in the Barking district of east London in connection with the attack and raids were continuing there, the force said.
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The attack occurred five days before a parliamentary election and was the third to hit Britain in less than three months. May said the vote would go ahead as planned on Thursday (local time).
"It is time to say enough is enough," the Conservative leader said in a televised statement outside her Downing Street office, where flags flew at half-mast.
"We cannot and must not pretend that things can continue as they are," May said, calling for a beefed-up counter-terrorism strategy that could include longer jail sentences for some offences and new cyberspace regulations.
Christchurch man Oliver Dowling was among those injured in the Borough Market stabbing - and he is now in an induced coma after lacerations to the face, neck and stomach.
His injuries are described as serious, though the stabs missed his vital organs.
"Oliver is now stable after a four-hour-long surgery. Doctors are very happy with how he's come out the other side," Mr Dowling's sister Freddy wrote on Facebook.
"A massive thanks to the University of London Hospital for their tireless efforts in helping my brother out!!"
Mr Dowling's girlfriend, Marie Bondeville, was also injured in the attack, according to Ms Dowling.
An MFAT spokesperson earlier told Newshub no New Zealanders were caught up in the attack.
Less than two weeks ago, a suicide bomber killed 22 children and adults at a concert by US singer Ariana Grande in Manchester in northern England. In March, in an attack similar to Saturday's, five people died after a man drove into pedestrians on Westminster Bridge in central London and stabbed a policeman.
On Saturday night, police shot dead the three male assailants in the Borough Market area near London Bridge within eight minutes of receiving the first emergency call shortly after 10pm.
Eyewitnesses described harrowing scenes as the attackers' white van veered on and off the bridge sidewalk, hitting people along the way, and the three men then ran into an area packed with bars and restaurants, stabbing people indiscriminately.
Accounts emerged of people trying to barricade themselves in a pub while others tried throwing tables and other objects to fend off the attackers.
May's government announced that a nationwide minute of silence would be held at 11am on Tuesday to pay respect to the victims of the attack and flags would remain at half-mast on government buildings until Tuesday evening.
At an apartment block in Barking, a resident told Sky News he heard controlled explosions early on Sunday morning as police gained access to the building. A Reuters photographer later saw four women being removed from the building, shielding their faces as they stepped into police vans.
There was no immediate claim of responsibility for the London Bridge attack.
Islamic State, losing territory in Syria and Iraq to an advance backed by a US-led coalition, had sent out a call on instant messaging service telegram early on Saturday urging its followers to carry out attacks with trucks, knives and guns against "Crusaders" during the Muslim holy month of Ramadan.
Islamist militants have carried out scores of deadly attacks in Europe, the Middle East, Africa, Asia and the United States over the past two years.
Reuters / Newshub.