LONDON (Reuters) - Britain's terror threat level will remain at "severe" after militants killed seven people and injured 48 in London, Prime Minister Theresa May said on Monday (June 5), describing the assault as an attack on the free world.

Additional security measures have been put in place, including at several bridges in central London, May said.

Three knife-wielding assailants rammed a van into pedestrians on London Bridge and stabbed others nearby.

"JTAC, the independent joint terrorism analysis centre, have confirmed that the national threat level remains at severe, that means that a terrorist attack is highly likely," she told BBC television after a meeting of the government's emergency committee.

"It is now clear that, sadly, victims came from a number of nationalities. This was an attack on London and the United Kingdom, but it was also an attack on the free world."

Government and local authorities are working closely with the police to guarantee security around the national election, and robust plans have been in place for weeks, May's spokeswoman said separately on Monday.

Britain will vote in a national election on Thursday, just days after the third assault in Britain in less than three months.

"There are plans in place for the general election. Police have been working closely with local authorities for several weeks on this. Those plans were developed with the threat level being at 'severe' so they are pretty robust," the spokeswoman told reporters.

"Police review security for all events but we have been working very closely with them and local authorities for some time."

Separately, earlier on Monday London's police chief said the three attacks carried out by Muslim militants in Britain in the last three months have been largely domestic plots and the majority of the threat facing the country is not directed from overseas,

"All the recent attacks I think have a primarily domestic centre of gravity," Metropolitan Police Commissioner Cressida Dick told BBC radio.

"There are in the five that we have foiled and these three recent attacks, in some of them there are undoubtedly international dimensions. We will always be looking to see if anything has been directed from overseas but I would say the majority of the threat that we are facing at the moment does not appear to be directed from overseas."