Open House London returns to the capital this weekend, marking its 21st anniversary by offering visitors the first chance to enter Number 10 Downing Street, the official residence of the UK Prime Minister.

The building's current occupant, David Cameron, hailed the initiative: "Open House London gives people the chance to explore some of London's most unique and celebrated buildings," he said. "As the official residence and office of British prime ministers since 1735, Number 10 Downing Street is a truly historic address. I'm delighted to be opening up the famous black door for the first time this year."

Open House was created in London 21 years ago by Open-City, a leading independent architecture education organisation. The Open House model invites everyone to experience, explore and understand the value of a well-designed built environment. This successful concept has been extended to events in other cities around the world including New York, Dublin & Galway, Tel Aviv & Jerusalem, Barcelona, Rome, Helsinki, Slovenia and Chicago.

Victoria Thornton, Founding Director of Open-City said: "Open House is a unique and vibrant annual event, offering people across London the chance to explore and learn about the capital's architecture. In 2012, 87% of participants said the Open House annual event was the best way see, explore and learn about the capital's architecture. This year we're putting together a great programme of open buildings, walks, tours and talks taking in everything from award-winning contemporary homes to major regeneration areas."

This year, more than 800 buildings, from private residences to world-famous monuments such as Battersea Power Station, will be open to all comers.

For a full list of venues, see the Open House London website.

Scroll down for images of a selection of buildings taking part this year.