An ambulance helicopter landed on Parliament Square, just outside the building.
The BBC said police believed there was a suspect vehicle outside Parliament but police did not confirm that report.
Amid confusing scenes, it appeared the incident may have unfolded in several locations, including on the busy Westminster Bridge where tourists take pictures of Big Ben and other attractions.
Reporters inside Parliament said a large number of armed police, some carrying shields, were pouring into the building.
United States President Donald Trump said he had been briefed on events in London.
The incident took place on the first anniversary of attacks on Brussels in Belgium. Reuters
Armed police being deployed outside the Houses of Parliament in central London yesterday.