Ceremonies in Ukraine, site of the Chernobyl plant, also marked the fire and explosion at the station's fourth reactor on April 26, 1986 -- the world's worst civil nuclear accident that sent radiation over most of Europe.
Television in Russia aired documentaries on the accident.
The anniversary had traditionally had the most impact in Belarus, the country worst affected by the catastrophe, with about one-quarter of its territory contaminated.
The liberal and nationalist opposition had long used the disaster as a rallying point against President Alexander Lukashenko. But Sunday's turnout exposed opposition divisions as Lukashenko tries to improve Belarus' image abroad.
The president, long accused in the West of crushing human rights, embarked Sunday on a trip to Italy, his first official visit to the West since the mid-1990s.
"This is one of the smallest attendances ever at a Chernobyl anniversary," said Anatoly Lebedko, veteran leader of the opposition United Civic Party. "Everyone seems very tired."
The annual Chernobyl commemoration in Minsk reached a peak on the disaster's 10th anniversary in 1996, when tens of thousands of protesters clashed with police in central Minsk.
Activists are now divided over whether to back the European Union's suspension of sanctions against Belarus -- including an end, for now, to an EU entry ban on Lukashenko, which was imposed in 2006 on charges that he rigged his re-election.
Alexander Milinkevich, who ran against the president in 2006, backs the EU policy, while more radical opposition figures say punitive measures should be restored to force faster change.
Viktor Ivashkevich, of the nationalist Belarussian Popular Front, linked the low turnout to "confusion in the opposition and in society at large.
"Old ideas are being torn down with nothing new to replace them," he said. "Some say we have to cooperate with authorities to make them become more democratic. Others call for the same fight as before. People don't know what to do."
Lukashenko has sought better ties with the West since quarrelling with traditional ally Russia over energy prices in 2007. The EU acted after authorities made concessions, including the release of detainees deemed political prisoners and the staging of an election deemed fairer than previous contests.
Lukashenko hopes to turn the visit to Italy into a triumphant emergence from isolation. His program includes an audience with Pope Benedict in the Vatican, which he hopes could lead to a papal visit and perhaps even a meeting in Belarus between the leaders of the Catholic and Orthodox churches.
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