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Metropolitan Police arrest 12 in connection with London Bridge attack, investigation 'progressing rapidly'

London's Metropolitan Police said on Sunday that 12 arrests had been made in the wake of last night's attack at London Bridge, which was deemed an act of terrorism.

Police flooded London streets after at least seven people were killed in an area teeming with revelers late Saturday. A van filled with three armed men mowed down pedestrians on London Bridge, with all three attackers shot dead by police within minutes of the attack, Britain's third major terror incident in as many months.

An outlet affiliated with Islamic State claimed the militant group was responsibility for the incident, but NBC was not immediately able to independently verify the claim of responsibility. Still, the ensuing chaos — in a country still shaken by last month's bombing at a Manchester concert as well as an attack outside Parliament in March— quickly reverberated around the world.

Among the 12 arrested in the Barking area of London were 7 women, according to an official Met Police statement that did not name the suspects. Officials warned of an increased police presence around the city, with investigators still chasing leads and potentially arresting others.

"The public can expect to see additional police – both armed and unarmed officers - across the Capital as you would expect in these circumstances," Assistant Met Police Commissioner Mark Rowley told reporters. "And our security and policing plans for events are being reviewed, the public will also see increased physical measures on London's bridges to keep the public safe."

The attack left nearly 50 people injured, and authorities say some of those injuries were "life threatening," suggesting that the death toll could rise. Rowley said that 21 of those injured in the attack are in a critical condition, with 36 hospitalized.

A British Transport Police officer and an off-duty Met Police official were among those seriously hurt, according to the Met Police statement, but are expected to survive. Rowley told reporters that the attackers appeared to be wearing explosive vests, making the need to use lethal force and multiple rounds "a matter of life or death."

Police urged the public to "remain calm but vigilant" as the investigation continued, and asked for the public's cooperation with the probe.

In response to the attack, Prime Minister Theresa May vowed a tougher response to the Islamic extremism that has inspired terrorism across Western Europe. Saturday's attack occurred just days ahead of a June 8 U.K. parliamentary election that has tightened unexpectedly in recent days, and weeks after a suicide bomber left more than 20 people dead at a concert in Manchester.

While May is expected to keep her majority, several polls have suggested her Conservative Party has slowly surrendered a once commanding lead.

On Sunday, May said the recent attacks are not directly connected, but that "terrorism breeds terrorism" and attackers copy one another. She said five credible plots have been disrupted since March.

"They are bound together by the single evil ideology of Islamist extremism that preaches hatred, sows division, and promotes sectarianism," she said. "It is an ideology that claims our Western values and freedom, democracy and human rights are incompatible with the religion of Islam."

—The Associated Press contributed to this article.