At least two people were killed and dozens injured in a major terror incident outside the British Parliament, said a Reuters report.

An assailant stabbed a policeman and was shot by police just outside Britain's parliament building in London on Wednesday in what police described as a "terrorist incident".

Amid confusing scenes, it appeared the incident may have unfolded in several locations, including on the nearby Westminster bridge where eyewitnesses said a car had crashed into pedestrians, sad the report.

Two people died in the incident, according to Sky News, but the total number of casualties was unclear.

A parliamentary official earlier said two people had been shot outside parliament and the building was in lockdown.

The House of Commons session was suspended and members of parliament who were in the chamber were told to remain there, Reuters said.

Police said they had been called to an incident at Westminster Bridge, officers were on the scene and it was being treated as a firearms incident.

The incident took place on the first anniversary of attacks by Islamist militants that killed 32 people in Brussels.

Reuters reporters inside the parliament building heard loud bangs and shortly afterwards saw two people lying on the ground in a courtyard just outside, within the perimeter of the parliamentary estate.