Egyptian state TV says there has been a large explosion in downtown Cairo, near the city's police headquarters.

The report says the blast, which happened early on Friday, sent smoke rising above the Egyptian capital and that a large number of ambulances were on the way to the scene.

It says there was gunfire in the area immediately after the blast, which was heard in several parts of the city.

The report comes on the eve of the anniversary of the start of the 2011 uprising that toppled Egypt's longtime autocratic ruler Hosni Mubarak.

A Muslim Brotherhood-led coalition plans protests after Friday prayers across the country as part of their near-daily demonstrations against the July overthrow of Islamist President Mohammed Morsi and the recent vote on the country's rewritten constitution.