Prime Minister Theresa May was chairing a meeting of the Government's emergency Cobra committee to discuss the immediate response to the bloody incident at the Palace of Westminster.
Mrs May ordered flags to be lowered to half mast over Downing Street as a mark of respect to the innocent people who lost their lives.
The Cobra committee brings together government ministers with senior officials of the emergency services and security and intelligence agencies.
Meeting in a briefing room of the Cabinet Office on Whitehall, Cobra co-ordinates the high-level response to serious incidents, and has previously gathered after terrorist atrocities, including the July 7 attacks on the London transport network and the murder of soldier Lee Rigby.
There was no immediate information on who was attending Wednesday night's meeting, which took place around four hours after the attack.
A Number 10 spokesman said Mrs May was being kept updated about the investigation.
"The thoughts of the Prime Minister and the Government are with those killed and injured in this appalling incident, and with their families," said the spokesman.
"The Prime Minister is being kept updated and will shortly chair COBR."
The White House said US President Donald Trump spoke with Mrs May to "offer his condolences on today's terror attack in London and his praise for the effective response of security forces and first responders.
"He pledged the full cooperation and support of the United States Government in responding to the attack and bringing those responsible to justice."
US Secretary of State Rex Tillerson said: "On behalf of the United States, I express my condolences to the victims and their families.
"The American people send their thoughts and prayers to the people of the United Kingdom.
"We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference."
Cobra is named after Cabinet Office Briefing Room A, in which it often meets. In recent times, Downing Street has begun referring to it as COBR, as it sometimes meets in Briefing Room B.
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