LONDON (AP) — British police arrested a dozen people Sunday in a widening terrorism investigation after attackers using a van and large knives turned a balmy evening of nightlife into a bloodbath and killed seven people in the heart of London.

Although the attackers were also dead, authorities raced to determine whether others assisted them, and Prime Minister Theresa May warned that the country faced a new threat from copycat terrorist attacks.

The county's major political parties temporarily suspended campaigning with only days to go before the general election. May said the vote would take place as scheduled Thursday because "violence can never be allowed to disrupt the democratic process."

The assault unfolded over a few terrifying minutes late Saturday, starting when a rented van veered off the road and barreled into pedestrians on busy London Bridge. Three men then got out of the vehicle with large knives and attacked people at bars and restaurants in nearby Borough Market until they were shot dead by police.

"They went 'This is for Allah,' and they had a woman on the floor. They were stabbing her," witness Gerard Vowls said.