Tony Blair appealed for the support of the country and said failure to act would amount to "unpardonable weakness and dereliction. This is simply the right thing to do."
In the five-minute broadcast, Mr Blair said: "It will be tough. But now that we have begun, I ask your support in seeing it through.
"We are doing what is right, for Britain, for Europe, for a world that must know that barbarity cannot be allowed to defeat justice."
Mr Blair focused on the acts of brutal repression by President Milosevic, which had led to 250,000 being made homeless and killed 2,000 since last spring. "Husbands taken from wives, fathers taken from children, never knowing if they are dead or alive... old women humiliated, young men massacred just for being Albanian, just for being there, when the Serb killing machine arrived."
Failure to act would mean "no justice, no protection" for them, he said.
After a day in which Nato blasted targets in the suburbs of Belgrade in its first daylight raids, the Prime Minister sought to counter public anxieties at the spectacle of Nato forces pounding sites in a familiar European landscape for the first time since the Second World War.
The broadcast marked an extension of the propaganda battle at home to shore up support for the raids and prepare public opinion for possible casualties when Mr Milosevic orders the use of his potent ground-to-air missiles against Nato air forces.
Underlining the efforts made by Nato to reach a peaceful settlement with Mr Milosevic before resorting to bombing, the Prime Minister accused the Serbian leader of being responsible for atrocities in Kosovo and for threatening to spread the chaos to other parts of Europe.
Earlier, Mr Blair told party supporters in Wales: "If we walk away from Kosovo and the plight of its people, it would be a betrayal of everything this nation stands for at its best."
Nato had taken action over Kosovo only "after the most careful consideration", the Prime Minister said.
"We went the extra mile for peace. Our demand on Milosevic is not great - indeed, it is very modest. It is to stop the brutality and ethnic cleansing. It is to treat the Kosovo Albanians as dignified human beings."Reuse content