[LONDON] British police have made seven arrests and searched six addresses after an attack near parliament which left four people dead and 29 injured, the country's top anti-terrorism officer said on Thursday.

"We have searched six addresses and made seven arrests. The inquiries in Birmingham and other parts of the country are continuing," Mark Rowley told reporters outside London's police headquarters.

British counter-terrorism police were investigating the background of the attacker.

The attack unfolded on Wednesday across Westminster Bridge in the shadow of Big Ben, a towering landmark that draws tourists by the millions and stands over Britain's Houses of Parliament - the very image of London.

The attacker's car struck pedestrians on the bridge before crashing into the railings surrounding the heavily guarded Houses of Parliament, sowing first shock then panic in the seat of British power.

The assailant then ran through the gates brandishing a knife and stabbed a 48-year-old policeman to death before being shot dead by another officer.

AFP,REUTERS