LONDON: Police specialists collected evidence in the heart of London after a series of attacks described as terrorism killed six people and injured more than 40 others. Prime Minister Theresa May called an emergency security cabinet session Sunday to deal with the crisis.

The assault began Saturday night when a van veered off the road and barreled into pedestrians on London Bridge. Three men fled the van with large knives and attacked people at bars and restaurants in nearby Borough Market, police and witnesses said. More than 40 people were treated at London hospitals and a number of others suffered less serious injuries.

London Mayor Sadiq Khan said Sunday that some of the injured were in critical condition.

The violence turned a summery night in an area packed with revelers into a scene of panic and chaos, with officers running through crowded streets screaming for people to flee. Lifeboats on the River Thames helped evacuate the area.

The Metropolitan Police force declared the attacks "terrorist incidents." Assistant Commissioner Mark Rowley said that while they believe all the attackers were killed, the investigation continued.

"We believe three people were involved, but we still have got some more inquiries to do to be 100 percent confident in that," he said.

As dawn broke over the capital, a large area on the south bank of the River Thames remained cordoned off. Police told people to avoid the area, leaving tourists and revelers struggling to get home.

The attack comes just days before Thursday's general election. May's Conservative Party had been expected to win by a wide margin but recent polls have showed the race tightening. The party suspended national campaigning Sunday because of the attack.

Khan said the attackers hoped to disrupt democracy and hamper plans for voting, but that would not happen. He said Londoners should remain vigilant but added: "I'm reassured we are one of the safest global cities in the world."

Only hours before, bursts of gunfire echoed through the streets — likely from armed police — and at least three blasts rang out as officers 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.

"The suspects were wearing what looked like explosive vests," Rowley said. "But these were later established to hoaxes."

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."

Police received the first call of reports of a vehicle hitting pedestrians on London Bridge at 10:08 pm (2108 GMT). Rowley said the attackers were dead in eight minutes.

Multiple witnesses reported a vehicle veering off the road and hitting as many as six pedestrians.

Soon after, reports started coming in of stabbings at Borough Market, a nearby area full of bars and restaurants surrounding a popular food market.

Photographer Gabriele Sciotto said he was on his way home from a pub when he saw a man running toward him telling him to turn around and run because there was a terrorist attack. As a documentary filmmaker, though, his instincts were to keep going.

The 25-year-old said that at first, "it didn't look too dangerous."

The men went into Borough Market, but then turned around and ran toward the Wheatsheaf Pub after being confronted by a police officer, he said, and suddenly a lot of police came from the other direction.

The attackers "had no clue what they were doing. They were scared. The police were scared," he said.

After one of the men was shot, Sciotto, captured a photo of the suspect on the ground wearing what appeared to be canisters strapped to his chest.

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."

May confirmed 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." Her office said she would chair a meeting of the government's COBRA emergency committee on Sunday.

As thousands of people flooded from the area of the attacks — many unable to get home with nearby subway stations shut — locals 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.

It was 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. After that attack, Britain's official threat level from terrorism was raised to "critical," meaning an attack may be imminent. Several days later it was lowered to "severe," meaning an attack is highly likely.

A charity concert for victims of the Manchester attack, featuring Grande and other stars, is due to be held in the northwest English city on Sunday.

Grande, who visited victims of the Manchester attack in the hospital on Friday, found herself confronted with another terror attack.

She tweeted: "Praying for London."