The Metropolitan Police has confirmed its Counter-Terrorism Command is leading the investigation after a car crashed into security barriers outside the Houses of Parliament.
It comes just 11 months after a partially exploded bomb was left on a tube train - the last major incident to rock Britain.
Here is a timeline of attacks in recent years:
- September 15 2017: A partially exploded device planted on a District line train left more than 51 people injured.
Ahmed Hassan, 18, was jailed for at least 34 years for planting the Parsons Green tube bomb which caused a huge fireball.
- June 19 2017: One man dies and several others are injured after a man rammed his van into worshippers in north London.
Darren Osborne, 47, of no fixed address in Cardiff - who had been radicalised by far-right material, was jailed for at least 43 years after being found guilty in February of murder and attempted murder.
- June 3 2017: Eight people are killed in a terror attack around London Bridge.
A van ploughed into people on the bridge before the three attackers carried out a knife rampage in Borough Market. The perpetrators - Khuram Butt, 27, Rachid Redouane, 30, and Youssef Zaghba, 22 - were shot dead by police.
- May 22 2017: Twenty-two people - including children - are killed in a bombing at a pop concert in Manchester.
Lone suicide attacker Salman Abedi detonated an explosive device as crowds of music fans, many of them youngsters, left Manchester Arena following a performance by US singer Ariana Grande.
- March 22 2017: Five people are killed in a car and knife attack in Westminster.
Khalid Masood drove a hire car over Westminster Bridge, near the Houses of Parliament, mounted the pavement and hit pedestrians before crashing into railings outside the Palace of Westminster.
He stabbed Pc Keith Palmer, 48, to death. Also killed in the atrocity were US tourist Kurt Cochran, Romanian tourist Andreea Cristea, 31, and Britons Aysha Frade, 44, and 75-year-old Leslie Rhodes. Masood was shot dead by police.
- June 16 2016: Labour MP Jo Cox is murdered outside her constituency office in Batley, West Yorkshire.
The mother-of-two, 41, was shot and stabbed multiple times by right-wing extremist Thomas Mair. He was later handed a whole-life prison sentence for her murder.
- May 22 2013: Fusilier Lee Rigby is murdered by Islamist extremists Michael Adebolajo and Michael Adebowale.
The 25-year-old serviceman was walking near his barracks in Woolwich, south-east London, when the pair rammed him with a car before attempting to hack off his head with knives. The killers were jailed at the Old Bailey in February 2014.
- July 7 2005: Four suicide bombers kill 52 and injure hundreds of others in blasts on the London Underground network and a bus.
Twenty-six died in the bombing at Russell Square on the Piccadilly line, six in the bombing at Edgware Road on the Circle line, seven in the bombing at Aldgate on the Circle line, and 13 in the bombing on a bus at Tavistock Square.