Radovan Karadzic has spent his first night in a cell at the special war crimes court in Belgrade, after 13 years on the run.
Mr Karadzic's lawyers may appeal to fight his extradition to the Hague
This was the arrest many people in the Balkans and beyond had come to believe would never happen.
Bruno Vekaric, the spokesman for the Serbian war crimes prosecutor, confirmed to the BBC that Mr Karadzic had been arrested on Serbian territory.
He added that all further information would only be released after a mid-morning meeting on Tuesday of the Action Team - comprising prosecutors, police and intelligence chiefs.
The statement from the Serbian president's office said that the Bosnian Serb wartime leader was arrested "in an action by the Serbian security services".
On a visit to the heavily-guarded special court last week for a background briefing on other war crimes issues, I asked in passing about Mr Karadzic and the other main fugitive, military leader Ratko Mladic.
Since the formation of the new government in Belgrade, I was told, there is a new atmosphere surrounding the fugitives.
What that probably meant in practice, is that those members of the Serbian security services who knew where Mr Karadzic was hiding, have either passed on that information, or simply stepped aside to allow pro-Western agents to arrest him.
The replacement of the old head of Serbian Intelligence, Rade Bulatovic, with a new man, Sasha Vukadinovic, must have played a significant role.
The appointment was only made public last week. Mr Vukadinovic established his credentials by engineering the arrest of one of Serbia's mafia groups, the so-called Jitka clan.
But there had been other, more mixed signals from Belgrade recently. Ivica Dacic, the interior minister in the new government is leader of the Socialist Party - established by the late President Slobodan Milosevic, who died in March 2006 in custody in The Hague, after a war crimes trial which dragged on for four years.
So there is little love lost between the Socialists and the international tribunal.
But in the end, insistence by several European governments, led most recently by the Netherlands, that Serbia's progress towards European Union membership could not proceed until "full co-operation" with the tribunal was guaranteed, appears to have had the desired effect.
Still at large
The chief prosecutor for the tribunal, Serge Brammertz, was already due to visit Belgrade in the coming days. He was among the first to welcome the arrest of Mr Karadzic.
"This is a very important day for the victims who have waited for this arrest for over a decade. It is also an important day for international justice, because it clearly demonstrates that nobody is beyond the reach of the law, and that sooner or later all fugitives will be brought to justice," Mr Brammertz said.
Only two men indicted by the Tribunal are still at large. Ratko Mladic, the Bosnian Serb military commander, who is accused with Mr Karadzic of genocide for the shelling of Sarajevo, and the massacre of Bosnian Muslim men and boys at Srebrenica. And Goran Hadzic, who is accused of war crimes against Croats in the city of Vukovar.
But many more war crimes cases will be handled by local courts, once The Hague tribunal winds up its work, in 2010.