A van ploughed into pedestrians and several people were stabbed in central London on Saturday, leaving at least 20 casualties in what police called a “terrorist” attack days before a general election.

Armed police opened fire during two “terrorist incidents” at London Bridge and nearby Borough Market, a popular nightspot teeming with bars on a warm summer evening.

“We can confirm we have taken at least 20 patients to six hospitals across London,” the London Ambulance Service said in a statement.

Witnesses described a van speeding into several pedestrians on London Bridge and then a knife-wielding man sprinting towards a bar packed with revellers enjoying a Saturday night out.

One witness described seeing five assailants and several people said they were locked inside pubs and restaurants by police and had to come out with their hands on their heads.

Italian photographer Gabriele Sciotto, who was watching the Champions League final at the Wheatsheaf pub in Borough Market, said he saw three men being shot just outside the pub.

In his picture, a man wearing combat trousers, with a shaved head and what looked like a belt with canisters attached to it could be seen on the ground with two more bodies behind him.

Pictures showed medical staff attending to apparent victims of the attack and a helicopter buzzing just metres over London bridge, one of the major arteries in the City business district.

Police responded rapidly with dozens of emergency vehicles and authorities led shellshocked members of the public away from the scene.

The attack came more than a week after a suicide bomber killed 22 people at a concert in the northern city of Manchester and days ahead of the June 8 election, where security is a major theme.

It also sparked harrowing memories of an attack on Westminster Bridge when 52-year-old British Muslim convert Khalid Masood rammed his car into pedestrians before crashing into the barriers surrounding parliament and then stabbing a police officer to death.

It is the latest in a string of attacks to hit Europe, including in Paris, Berlin and Saint Petersburg.