Air raid sirens have sounded and church bells have rung across Serbia as the country marks 10 years since the start of Nato's bombing campaign.
Serbs have been gathering at sites where people were killed and government ministers were expected to lay wreaths.
Nato bombed Serbia for 11 weeks in an effort to push Serbian forces out of the province of Kosovo, accusing them of atrocities against ethnic Albanians.
Hundreds of Serbs were killed as bombs struck military and civilian targets.
Damaged buildings can still be seen in the capital, Belgrade, and in Pristina, capital of the self-declared independent state of Kosovo.
The strikes were the largest military operation ever undertaken by Nato, and the first time it had used force against a sovereign state without UN approval.
'Tragic days'
At a special cabinet meeting, Serbian Prime Minister Mirko Cvetkovic said that 2,500 civilians, including 89 children, along with 1,002 soldiers and policemen had been killed during the Nato offensive.
For the sake of the future of our children, we must not allow something like this to be repeated ever again
Human Rights Watch puts the civilian death toll at about 500.
"The attack on our country was illegal, contrary to international law, without a decision by the United Nations," Mr Cvetkovic said. "Serbia cannot forget those tragic days."
He added: "The air strikes have not solved problems in Kosovo, and did not help to bring peace and the rule of law.
"On the contrary, they resulted in ethnic cleansing and gross violations of human rights, international standards and fresh tensions."
Mr Cvetkovic said that for the sake of their children, Serbs "must not allow something like this to be repeated ever again".
Serbs are feeling a mixture of anger and sadness over what they feel was Western aggression, says the BBC's Helen Fawkes in Belgrade.
Many of Serbia's roads, bridges, military bases and weapons plants were bombed until 10 June 1999, when an agreement was reached for Yugoslav forces to withdraw from Kosovo.
The withdrawal paved the way for Nato peacekeepers and a UN civilian mission to run the province, which unilaterally declared independence from Serbia in February 2008.
The strikes were the largest operation ever undertaken by Nato
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