Over 50 campaigners occupied the offices of security firm G4S last night, in protest at the firm's supply of equipment to Israeli forces in Gaza.
The British-Danish company came to public attention when it failed to provide security personnel to the Olympics.
It is often the subject of protests, especially after the death of Jimmy Mubenga, an Angolan asylum seeker to the UK who died after being restrained by its guards.
"G4S is an active accomplice to Israel's collective punishment of Palestinians in Gaza," said Bob Williamson of BDS, a Palestinian solidarity group in London.
Fellow campaigner Maham Hashmi added: "How can the UK government give lucrative public contracts to a company that shows such disregard for basic human rights standards across all of its business activities?"
The protestors entered the Victoria building at 16:00 GMT yesterday before staging a sit-in protest and locking themselves together. Police removed them from the premises after about an hour.
G4S has contracts with various Israeli agencies to provide equipment and services to Israeli checkpoints into the Gaza Strip. It also has a contract with the Israeli Prison Service.
Over 105 Palestinians have been killed since the start of the Israeli strike on Gaza last week, in which it assassinated Hamas military commander Ahmed Jabari.
Last night there was a last-minute bombardment of Gaza ahead of an expected ceasefire, while 150 rockets were fired back at Israel.
At least 30 Palestinians were killed.