These are designed to convey the message that delaying intervention will cost lives in the troubled western region of Sudan.
The focus of the day was in London, where hundreds of demonstrators marched on to Downing Street, the office of Tony Blair, Britain's prime minister, to hand in a letter to a government minister.
Gareth Thomas, a government minister with responsibility for international development, calling for the quick deployment of a strong peacekeeping force in the region of western Sudan.
The letter, addressed to Blair, urged the prime minister "to use your influence to push the international community to call for action".
"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 peacekeeping force on the ground protecting civilians," the letter said.
Activists also erected a two-metre-high hourglass.
Organisers said 3,500 people attended the rally. Police did not immediately have an estimate of the turnout.
Rome and Berlin
In Rome, hundreds of people took part in a march on the Colosseum.
There was also a 200-strong protest in Berlin, where marchers carried alarm clocks and a banner saying: "It is five minutes to midnight, we're sounding the alarm!"
To coincide with the day, actors and musicians including Elton John, George Clooney, Bob Geldof, Mick Jagger and Hugh Grant released a statement calling on the world's most powerful countries to take "decisive action" over the atrocities.
Amnesty International attacked the "vicious crimes against humanity" which it said the Sudanese government has allowed to take place in Darfur.
"Enough is enough. It is time for the rest of the international community to take effective action," Kate Allen, Amnesty's UK director, said in a statement.
Demonstrations were planned in more than 30 countries on Sunday, designated by campaigners as a global day of action against the conflict that has killed at least 200,000 people and displaced more than 2.5 million since 2003.
Organisers said they chose the date, April 29, as the approximate fourth anniversary.
Sudan contests the figures, saying that only 9,000 people have died.
The US and Britain have in recent weeks indicated that their patience with Omar al-Bashir, Sudan's president, is running out.
Although al-Bashir recently agreed to let 3,000 UN troops in to boost the weak African Union force in Darfur in a first phase deployment, he has objected to some elements of the larger operation.
These include allowing the UN to share command of the forces with AU counterparts.