LONDON — Terrorism struck at the heart of London, police said Sunday, after a vehicle veered off the road and mowed down pedestrians on London Bridge and witnesses told of men with large knives stabbing passersby at nearby Borough Market.

British police said six people were killed, in addition to three people identified as the attackers. The London Ambulance Service said at least 30 people had been taken to five hospitals.

The Metropolitan Police force declared the attacks “terrorist incidents.”

The violence turned a summery Saturday night in an area packed with bars and restaurants into a scene of panic and chaos, with officers running through crowded streets screaming for people to flee and lifeboats drafted to help clear the area.

Hours after the attacks began, a large area of central London remained cordoned off and police told people to avoid the area, leaving tourists and revelers struggling to get home. It was unclear whether any of the attackers were on the run.

Bursts of gunfire echoed through the streets — likely from armed police — and at least three blasts rang out as police performed controlled explosions. One image taken by a witness showed a man on the ground surrounded by police; he appeared to be wearing a vest with canisters attached to it.

Gerard Vowls told The Guardian newspaper that he saw a woman being stabbed by three men at the south end of London Bridge. He said he 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 newspaper. “I want to know if this girl is still alive. I've been walking around for an hour and a half crying my eyes out. I don't know what to do.”

Bartender Alex Martinez said he hid in a garbage bin for a half-hour when a man stormed the restaurant where he worked, which was nearly full. “I saw that man with a knife in his hand and after that a man started screaming so I knew something wrong was happening,” he said.

Medics treated the injured near the market as shocked people cried and shouted around them. Police officers yelled at people to run from the area.

Simon Thompson told Sky News that he was just outside Borough Market when he saw crowds fleeing. “We ran for like 100 meters and then saw loads of police cars turned up and there was kind of a period of quite intense gunfire,” he said. “I hid in a restaurant basement for about an hour. Police told us to get out and then there was more gunfire.”

The mayhem began just after 10 p.m. local time, when police responded to reports of a vehicle hitting pedestrians on London Bridge, which crosses the River Thames in central London. Multiple witnesses reported a vehicle veering off the road and hitting as many as six pedestrians.

“We saw injured people on the road, injured people on the pavement,” witness Will Heaven told Sky News.

Soon after, reports started coming in of stabbings at Borough Market, a nearby area full of bars and restaurants surrounding a popular food market. Witnesses reported seeing as many as three attackers with knives.

Police initially said officers were also responding to a third incident, in the Vauxhall neighborhood, but later said that turned out to be an unrelated stabbing.

Police tweeted a warning telling people in the area to run to safety, hide and then call police if it is safe to do so. They asked Londoners to “remain calm but be alert and vigilant.”

Prime Minister Theresa May confirmed that the attack “is being treated as a potential act of terrorism” and said her thoughts were with “those who are caught up in these dreadful events.” May's office said she would chair a meeting today of the government's COBRA emergency committee.

As thousands of people flooded from the area of the attacks — many unable to get home with nearby subway stations shut — local residents were quick to offer assistance.

The Royal Oak pub, near the area of the attacks, opened its doors to people evacuated from hotels. At least one taxi company offered free rides to people stranded in the area.

Phaldip Singh, who describes himself as an entrepreneur and youth activist, tweeted that Sikh temples were open to provide food and shelter for those affected.

If confirmed as terrorism, this would be the third attack to hit Britain in as many months. In March, a British convert to Islam ran down people with a vehicle on Westminster Bridge, killing four, then stabbed a policeman to death outside Parliament.

On May 22, a British suicide bomber killed 22 people and injured dozens at an Ariana Grande concert in Manchester. A charity concert for victims of the Manchester attack, featuring Grande and other stars, is due to be held today.

This spring the U.S. Transportation Security Administration issued a report titled “Vehicle ramming attacks: Threat landscape, indicators and countermeasures,” which notes that in the last three years, at least 173 people have been killed and more than 700 wounded in 17 ramming attacks around the world.

The London attack comes at a politically sensitive moment. On Thursday, British voters are set for a general election.

The U.S. State Department said the “United States condemns the cowardly attacks targeting innocent civilians in London this evening.”

State Department spokeswoman Heather Nauert said Saturday evening: “The United States stands ready to provide any assistance authorities in the United Kingdom may request.”

President Donald Trump argued in favor of his controversial travel ban as news of the London attacks spread. One tweet read: “We need to be smart, vigilant and tough. We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!”

A few minutes later Trump tweeted a message of support for Londoners: “Whatever the United States can do to help out in London and the U. K., we will be there - WE ARE WITH YOU. GOD BLESS!”

Earlier this week, the Trump administration asked the Supreme Court to immediately reinstate its ban on travelers from six mostly Muslim countries and refugees from anywhere in the world.