BELGRADE -- Serbia's presidential elections on Friday became a race between the two main reformist camps which ousted former strongman Slobodan Milosevic two years ago, as Yugoslav President Vojislav Kostunica announced his candidacy.

Ending weeks of speculation, Kostunica said he would be running in the elections if he received the backing of his Democratic Party of Serbia (DSS).

The move sets the scene for a showdown with Yugoslav Deputy Prime Minister Miroljub Labus, who has the open support of Serbian Prime Minister Zoran Djindjic -- Kostunica's main rival for power in the republic, AFP reported.

It will also lead to open confrontation between the more conservative Kostunica and the pro-West Djindjic, which could complicate key decisions on economic revival, cooperation with Europe and the proposed union with Montenegro to replace the former Yugoslavia.

Labus, a leading advocate of reforms in the past two years, has already launched a campaign focusing on Serbia's integration to Europe and economic reform deemed crucial to attracting foreign investment.

"Serbia is at a crossroads. If I am elected, that would mean that Serbia has decided to continue along the same path, the path of reform," Labus told supporters this week.

Kostunica's announcement has stamped out any hope that the reformist camp, which overcame internal differences to topple Milosevic in October 2000, might find a common vision for the future of the impoverished country as it struggles to emerge from sanctions and international isolation.

On the surface the race is between Kostunica, a 58-year-old jurist, and Labus, a 55-year-old economist well-regarded in Western Europe.

But eventually it will be seen as a battle between Djindjic and Kostunica, key figures in the ruling dos coalition whose paths took radically different directions following Milosevic's ouster.

Kostunica accuses Djindjic of trampling on the constitution in a bid to grab "absolute power" for himself and his close circle of supporters.

The two men rarely talk to each other, and Kostunica's DSS recently walked out of the Serbian Parliament in protest at alleged corruption.

Kostunica confirmed Friday he would push for new parliamentary elections if he became president, accusing the Serbian government of encouraging "mafia elements".

"There is no development without the polls. That is why I will run, fully aware that this time there will be no cohabitation with the rest of the Dos, but a confrontation," Kostunica warned.

"My victory at the elections in September would mark the start of a fight for the rule of law, democratization of Serbia and a joint state of Serbia and Montenegro.

The rivalry between the two men has only worsened since Milosevic's extradition to the un war crimes court in June 2001, which was ordered by Djindjic despite Kostunica's protests.

Kostunica's decision to run for the top job in Serbia could have been motivated in part by his concern over the future union of Serbia and Montenegro, which could see the Yugoslav president's powers greatly reduced.

He has been a strong advocate of creating a robust federation between the neighboring republics, despite Montenegro's push for a loose union followed by independence after three years.

Since the wars that tore it apart in the 1990's, Yugoslavia has been reduced to only two republics, Serbia and Montenegro.

The four other original republics -- Bosnia-Hercegovina, Croatia, Montenegro and Slovenia -- have become independent states, while Kosovo, an Albanian-majority province of Serbia itself, has been placed under UN administration.