'Blood' flowed from a huge hourglass as thousands gathered opposite Downing Street for an international day of action marking the fourth anniversary of the start of the conflict in Darfur.
The two-metre-high hourglass was the centrepiece of the event to demand protection for civilians.
More than 200,000 people are estimated to have died in the bloodshed.
Over 10,000 hourglasses were being turned over by activists around the world during Global Day for Darfur events.
Campaigners said this underlines that the time is up to fail to protect the people of Darfur.
A letter addressed to Prime Minister Tony Blair was handed in to Number 10, before the protest.
It read: "Time is running out for the people of Darfur, and we urge you to keep the pressure on the government of Sudan until there is an effective peace-keeping force on the ground protecting civilians.
"Whilst we welcome the efforts that you and your government have taken, there is still an urgent need to do more.
"We therefore urge you to use your influence to push the international community to call for action."
The letter called for the fastest means possible for the implementation of a heavy support package. It also called for effective peacekeeping by increasing pressure on the government of Sudan to consent to the rapid deployment of an effective peacekeeping force.
