Seven people were killed and at least 48 injured in terrorist attacks in central London on Saturday, British authorities said, prompting Prime Minister Theresa May to vow a tough response to Islamist extremism.

A van was driven at pedestrians on London Bridge shortly after 10 p.m. Saturday, before three assailants emerged from the vehicle and went on a stabbing rampage at nearby Borough Market. Police shot and killed all three assailants, they said.

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Twelve people were later arrested in Barking, east London, and several locations were searched in connection with the attack, the police said.

In televised remarks on Sunday morning, the prime minister said, "It is time to say enough is enough," following deadly terrorist attacks near the Houses of Parliament in London in March and a fatal explosion at a crowded concert hall in Manchester in May.

The prime minister, who had just chaired an emergency meeting of government officials, called for a "robust" counterterrorism strategy, including longer sentences for terrorism-related crimes. "We cannot and must not pretend that things can continue as they are. Things need to change."

She added that the two major political parties had suspended campaigning but will resume Monday, and that the general election will take place as scheduled on Thursday.

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The attacks on Saturday came after the country's terrorism threat level was returned to "severe" on May 27 after being raised to "critical," the highest level, for the first time in a decade following the deadly explosion at the concert hall in Manchester on May 22.

After the latest incidents, May spoke on the phone with U.S. President Donald Trump, who offered his condolences, according to the White House.

Trump also offered the full support of the U.S. government in investigating and bringing those responsible for the attacks to justice, it said.

Heather Nauert, a State Department spokesperson, also denounced the attacks in a statement, categorizing them as "cowardly attacks targeting innocent civilians." Pledging to provide support to Britain if necessary, the statement said "all Americans stand in solidarity" with Britain.

Other countries also condemned the terrorist attacks and offered condolences.

Japanese Prime Minister Shinzo Abe sent a condolence message addressed to May, expressing Japan's solidarity with the people of Britain and vowing to combat terrorism in close cooperation with Britain and the international community.

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French President Emmanuel Macron tweeted his condolences to the victims and their families, while stressing his country stands by Britain.

Russian President Vladimir Putin called for joint efforts in fighting terrorists all over the world in his message of condolence to May, his office said.

At Borough Market, eyewitnesses reported police had fired their weapons, with the attackers who arrived in the van the likely targets.

The police sealed off the bridge and underground train stations around the area.

In March, four people were killed and 40 injured near the Houses of Parliament in London when an attacker plowed a car into pedestrians. The suspect was shot dead by the police and the Islamic State militant group later claimed responsibility.

In the Manchester attack, at least 22 people, including children, were killed and nearly 60 people injured in the blast by a suicide bomber at a concert of U.S. pop singer Ariana Grande.

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