They demanded it be transferred to the people of Libya.A Scotland Yard spokesman told AFP: "Police were called to an address in Hampstead Garden Suburb at approximately 12:50 pm local time today to reports of unknown people entering the premises.
"At this time this is being treated as a civil matter. There have been no arrests.
"Police are monitoring the situation and safer neighbourhood reassurance patrols are taking place in the area."
The house, in a secluded, leafy close surrounded by some of Britain's most expensive residential streets, boasts an indoor swimming pool, a sauna room and a cinema room.
"We do not trust the British government to return it to the people of Libya because they have a long history of helping the Gaddafi regime," "Topple the Tyrants" spokesman Montgomery Jones told the Hampstead and Highgate Express local newspaper.
"When we found out one of the world's most brutal dictators owned property in north London it seemed like an obvious thing to occupy it for the people of Libya.
"Our aim is to make sure that the assets stolen by Gaddafi are returned to the Libyan people and don't disappear into the pockets of governments or corporations.
"We want to welcome refugees from the conflict in Libya and those fleeing tyranny and oppression across the world."
Seif, who studied at the London School of Economics, bought the house in 2009.
It was put up for sale again at a price of 10.95 million pounds as the Libyan crisis unfolded, though it has since been taken off the market.
Mike Freer, who represents the local Finchley and Golders Green area in parliament, has called for the house to be seized and for the extended Gaddafi family to be barred from entering Britain.
"This house and any other assets in the UK should be seized until the provenance of the family's funds can be determined," he said