The co-ordinated demonstrations come ahead of a meeting this week of the UN general assembly.
Organisers, who planned protests in more than 30 countries, said some in the international community had become complacent since the UN Security Council approved plans in July for a 26,000-strong peacekeeping force for the vast region.
The deployment of the joint African Union-United Nations peacekeeping force faces delays, however, due to a lack of aviation, transport and logistics units, Ban Ki-moon, the UN secretary-general, said last month.
In the meantime Darfur's violence has increased, organisers said.
Campaigners are demanding that the force be deployed quickly, and that the world's most powerful countries put pressure on all sides in the conflict until attacks on civilians stop.
In London, demonstrators carried signs reading 'Stop genocide in Darfur' and 'Rape, torture, murder. How much longer for Darfur?'
In a video made to mark Darfur Day, Desmond Tutu, the South African archbishop, called Darfur "the world's largest concentration of human suffering", adding "it's also entirely avoidable if people speak out".
More than 200,000 people have died and 2.5 million have been uprooted since ethnic African rebels took up arms against the Arab-dominated Sudanese government in 2003, accusing it of decades of neglect.
Sudan's government is accused of retaliating by unleashing a militia of Arab nomads known as the Janjawid - a charge it denies.
Sunday's events were being organised by a coalition of more than 50 organisations including Amnesty International, Human Rights Watch, and the Save Darfur Coalition.