Different photographers captured at least a dozen people injured on the ground and photographs showed people lying on the ground, bleeding heavily. The number of casualties was unclear as at this moment.
The House of Commons, which was in session at the time, was immediately suspended and lawmakers were asked to stay inside.
Prime Minister Theresa May was safe after the incident, a spokesman for her office said. He declined to say where May was when the attack took place.
The leader of the House, David Lidington, said in the chamber that an assailant who stabbed a policeman had been shot by police.
Amid confusing scenes, it appeared the incident may have unfolded in several locations, including the busy bridge where tourists often congregate to take pictures of Big Ben and other attractions.
The incident took place on the first anniversary of attacks on Brussels in Belgium.
Reuters reporters inside parliament said a large number of armed police, some carrying shields, were pouring into the building.
Britain is on its second-highest level of “severe” meaning an attack by militants is considered highly likely.
In May 2013, two British Islamists stabbed to death soldier Lee Rigby on a street in southeast London.
In July 2005, four British Islamists killed 52 commuters and themselves in suicide bombings on the British capital’s transport system in what was London’s worst peacetime attack. (Reporting by Toby Melville and UK bureau, writing by Estelle Shirbon, editing by Stephen Addison)
This is a developing story.