They demanded that President Slobodan Milosevic should come to the aid of his Serb brethren and stop playing political games with the West.
The most vehement criticism came from Vojislav Seselj, self-proclaimed leader of the Chetniks, extreme Serb nationalists. Mr Seselj was released only three days ago from a Belgrade prison, and lost no time in convening a public meeting to call for action.
"Franjo Tudjman carried out the attack on the west of Krajina in agreement with Slobodan Milosevic," Mr Seselj charged on Friday. He claimed the Serbian and Croatian leaders had concluded a secret deal under whose terms the western Krajina would be sacrificed to preserve Serb possession of Eastern Slavonia, a fortified enclave of internationally recognised Croat territory north of Belgrade which abuts on Serbia's frontier.
Heavy exchanges of artillery fire broke out between rebel Serbs in Eastern Slavonia and the Croatian army yesterday, but there was no sign of movement by either side on the battlefield.
Mr Seselj said it would "be a logical response" for Serbs in Eastern Slavonia to launch an offensive against the Croat-held town of Osijek, and that "Serbs should bombard Zagreb".
Mr Seselj's Serbian Radical Party has long proclaimed a virulent brand of nationalism. Its most potent medicine is the undiluted mythology of Serb unity, a dogma to which many in Serbia still unfailingly subscribe.
Mr Milosevic retains complete political authority, but it would suit his purposes to appear vulnerable to extremist criticism. The fact that Mr Seselj's rantings were reported by Serbian press and television indicated the President thinks it wise to let off some steam, thus indicating to the West that he may face pressure to abandon his posture of "peaceful diplomacy".
A clue to the extremist agenda came from Mr Seselj when he said that if Mr Milosevic did not want to fight for the western Krajina he should at least "abolish the criminal blockade on the Drina and stop making things difficult for the Serbs in Krajina and Bosnia".
The Serbian blockade of the frontier - a porous and partial exercise at best - remains Mr Milosevic's chief weapon to wield pressure against the Bosnian and Krajina Serbs and to show political goodwill to the West.
Among the hardline Serb nationalists there is now a belief that Mr Milosevic may have sacrificed his status as the paramount Serbian political figure. Ludicrous though it may sound, some hope that the Bosnian Serb leader, Radovan Karadzic, could snatch the crown of Defender of the Serbs from the Belgrade apparatchik.
Mr Karadzic himself, who is estranged from Mr Milosevic, yesterday issued a cunning invitation to the Serbian leader to fulfil the destiny which many expect of him. "The people are asking themselves why has the President of Serbia, Slobodan Milosevic, given up on the national programme?" Mr Karadzic said. "I am now calling on him to resume the correct path and to resume the role of leader of all the Serbs."
The political balance in Belgrade is still tilted firmly in favour of Mr Milosevic. But a prolonged Croatian campaign, coupled, no doubt, with atrocity stories and the exodus of Serb refugees, could alter the equation. If the Serbian leader did indeed make a deal with Mr Tudjman, he must be hoping that Croatia will perform its dirty work quickly.