Prime Minister Theresa May said on Sunday the U.K. would review its anti-terrorism strategy in the wake of Saturday night's terror attack that left seven people dead and 48 others injured.
May's statement came following a high-level security meeting, also known as COBRA, in the capital.
"It is time to say enough is enough," May said.
A police statement later said 12 suspects had been arrested after raids were carried out in Barking, East London, in connection with last night's incidents. "Searches of a number of addresses in Barking are continuing," it added.
At least seven people were killed in attacks late Saturday that involved a van mowing down pedestrians on London Bridge; assailants also stabbed victims at nearby Borough Market.
May said: "Last night, our country fell victim to a brutal terrorist attack once again...Shortly before 10:10 yesterday evening, the Metropolitan Police received reports that a white van had struck pedestrians on London Bridge.
"It continued to drive from London Bridge to Borough Market, where 3 terrorists left the van and attacked innocent and unarmed civilians with blades and knives.
"All 3 were wearing what appeared to be explosive vests, but the police have established that this clothing was fake and worn only to spread panic and fear."
The premier also confirmed the death toll. "Seven people have died as a result of the attack, in addition to the 3 suspects shot dead by the police," she said.
"Forty-eight people are being treated in several hospitals across London, many have life-threatening conditions," she added.
Reminding that last night's attack was the third major terror attack in the past three months, the British prime minister said security and intelligence agencies and police disrupted five credible plots since the Westminster attack in March.
British national Khalid Masood was identified as the assailant in the March attack during which he too mowed down pedestrians on Westminster Bridge before attacking police outside the parliament building. Three civilians were killed on the bridge and one police officer was stabbed to death outside the parliament. Dozens of people were also injured in the attack. Masood was shot dead moments after the attack.
Also, 22 people were killed when suspected suicide bomber Salman Abedi detonated a device on May 22 in the foyer area of the Manchester Arena at the end of a pop concert; 59 others were injured. The terror threat level was upgraded following that attack to "critical", which meant a terror attack was imminent, but it was later downgraded to "severe" after a few days.
"We need to become far more robust in identifying it [terror threat] and stamping it out -- across the public sector and across society," May said.
Meanwhile, all political parties suspended their election campaigns on Sunday; however, May said full campaigning would resume on Monday and the general election would go ahead as planned on Thursday, June 8.
