Clashes between Egyptian security forces and more than 5,000 protesters in Cairo's Tahrir Square have left more than 1,036 people injured, according to witnesses and medical officials.

Tahrir Square, the epicentre of protests that toppled Hosni Mubarak, Egypt's former president, was sealed off early on Wednesday as lines of security forces in riot gear strived to regain control from demonstrators.

Witnesses said the clashes started on Tuesday when police tried to clear a sit-in at the state-TV building, which included families of those killed during the country's revolution earlier this year, known as the "martyrs", according to the Daily News, an Egyptian news website.

Witnesses said police attacked the families outside the Balloon Theatre in Agouza, where a planned memorial service for them was taking place.