"It is time to change the narrative," the letter reads. "It is time to tell a different story. This April many children in Darfur will be reaching their fifth birthdays without ever having known peace. The world needs to wake up."
British lawmakers including Nick Clegg, leader of the opposition Liberal Democrats, will join a rally in central London on Sunday. Protesters plan to gather outside the Sudanese Embassy.
A delegation of young survivors of the conflict will also hand in a series of drawings by Sudanese children, which depict the conflict, to employees at Prime Minister Gordon Brown's official residence.
Actors Matt Damon and Thandie Newton were also among those supporting the campaign worldwide, in a television advertisement campaign.
"After the genocide in Rwanda we all shook our heads and said 'never again,"' Damon said in a statement. "Today, as killings mount in Darfur we need to make 'never again' a reality and demand protection for the most vulnerable."
More than 200,000 people have been killed in the Darfur conflict, and over 2 million are displaced from their homes, the U.N. has said.
Fighting has raged in Darfur since 2003, when ethnic African tribesman took up arms, complaining of decades of neglect and discrimination by the Sudanese Arab-dominated government. Khartoum has been accused of unleashing janjaweed militia forces to commit atrocities against ethnic African communities in the fight with rebel groups.
Protests will take place in around 30 countries on Sunday, humanitarian charity Crisis Action said, calling for an immediate deployment of more peacekeeping troops.
U.N. Secretary-General Ban Ki-moon has been critical of slow progress in getting an expanded African Union-U.N. force in place.
The AU-U.N. force is authorized to have 26,000 troops and police, but Ban said only about 7,500 military personnel and 1,500 police officers were in Darfur on Jan. 31.
