The Prime Minister announced the step after chairing a meeting of the Government's Cobra emergency committee.
She said: "The police have put additional security measures in place to protect the public and provide reassurance.
"This includes additional security measures at a number of bridges in London."
Mrs May also confirmed the UK's terror threat level will remain at 'severe', meaning an attack is considered highly likely.
The level was raised to 'critical' after last month's Manchester terror attack but was lowered days later.
The PM said: "The independent Joint Terrorism Analysis Centre have confirmed that the national threat level remains at severe."
Theresa May said the UK's terror threat level remains at 'severe' London terror raids Sun, June 4, 2017
Police have raided a block of flats in Barking in connection with last night's London Bridge terror attack.
Getty Images 1 of 20
As police continue their investigations following the June 3 terror attacks, a block of flats in Barking is raided for possible suspects
The police have now identified all three of the attackers
Speaking to Sky News inside Downing Street, Mrs May added: "The police have now identified all three of the attackers.
"When progress in the investigation permits, the Metropolitan Police will release the names.
Get Quotes on Home Insurance
"Eleven individuals are being held in police custody following the search of two premises.
Police have installed concrete barriers on Westminster Bridge
"The police are working hard to establish the identities of all those who were tragically killed or injured in the event on Saturday.
"It is now clear that sadly, the victims came from a number of nationalities.
"This is an attack on London and the United Kingdom, but it is also an attack on the free world."
The COBRA briefing in Whitehall was attended by members of the Cabinet as well as London mayor Sadiq Khan.