Britain is investigating whether Facebook, the world's largest social media network, did enough to protect data.
The London offices of data analytics firm, Cambridge Analytica, have been raided by investigators from Britain's data watchdog.
About 20 officials entered the building after they were granted a warrant as part of an investigation into alleged misuse of personal information.
The officials, who were let into the building by security guards, could be seen checking books and papers through the windows of the second-floor offices on London's busy New Oxford Street.
The firm is at the centre of a storm over allegations it improperly harvested data from some 50 million Facebook users to target U.S. voters during the 2016 US presidential campaign.
Authorities in the United States and Britain are investigating whether it and Facebook broke the law in their use of personal information.
U.S. lawmakers on Friday asked Facebook Chief Executive Mark Zuckerberg to come to Congress to explain to explain how the data got into Cambridge Analytica's hands, adding to pressure on the firm, which is under fire from investors and advertisers.