A police officer is thought to have been stabbed by an assailant who was then shot by armed units on the parliamentary estate.
A car is also believed to have been driven into a crowd of pedestrians on Westminster Bridge.
MPs were on lockdown in the Commons and the entire complex is now in the process of being evacuated.
Prime minister Theresa May has also been evacuated to a safe location.
Police have appealed for members of the public who may have filmed the incident to share their footage with investigation teams. Visit http://ukpoliceimageappeal.co.uk/CSIPS_Public/upload/choose to upload.
Scarborough MP and immigration minister Robert Goodwill said he was coming out of the voting chambers when the lockdown was put in place.
He said: "I'd just finished voting and was heading to my office to await the next vote when we were put into lockdown.
"We were told there had been some kind of incident and were told Parliament had been suspended.
"We have now been locked into our offices while the police investigation is ongoing.
"My thoughts are obviously with the police officer and, from the reports we are getting, the other people who may have been caught up in this."