Updated

Populist leader Aleksandar Vucic was sworn in as Serbia's president on Wednesday, pledging to work for peace and stability in the war-weary Balkans while strengthening the country's armed forces.

Hours before the oath in parliament, Vucic's supporters clashed with opposition groups that were prevented from protesting in downtown Belgrade. Opposition groups said there were no injuries.

Later on Wednesday, a few thousand people joined an opposition protest against Vucic. Carrying banners that read "not our president" or "against dictatorship," opposition supporters marched through the Belgrade city center, blocking traffic while chanting slogans against Vucic and blowing whistles.

"There are two roads for Serbia: one led by Vucic, which is the road of dictatorship and destruction of freedom, and the road of freedom and democracy cherished by the people who gathered here today," said liberal leader Sasa Jankovic, who trailed Vucic at the presidential vote last month.

No incidents were reported.

Vucic, a former ultranationalist turned self-declared pro-European Union reformer, formally stepped down on Wednesday from his prime minister's post after winning the presidential election by a landslide in April.

The opposition has alleged irregularities during the election campaign including muzzling the media and accuses Vucic of autocratic rule.

"We must create a region where differences are an advantage and not the basis for clashes," Vucic said in his inaugural speech.

"There have been enough burials in the Balkans, it is time to give birth," he added, referring to the wars in the southern European region in the 1990s in which he supported the expansion of Serbia's borders at the expense of its neighbors.

Vucic also said Serbia won't join NATO nor become a member "of any other military alliance."

"While we have to talk to everyone, we also have to strengthen our defense capacity," he said.

During three years as prime minister, Vucic repeatedly called EU membership Serbia's "strategic goal." At the same time, he has been strengthening ties with Serbia's traditional ally, Russia, which has agreed to supply fighter jets and battle tanks to its military — heightening tensions in the Balkans.

Vucic only briefly mentioned Serbia's integration into the EU, noting that "no one should doubt Serbia's EU path."

He said the country will cherish good ties with Russia and China, but that it also should improve ties with the United States.

Addressing thousands of his cheering supporters in front of parliament, Vucic promised to "keep peace and stability, guard and protect the future of our children."

Serbia's presidency is largely a ceremonial post, but Vucic indicated he intends to continue playing a leading role in Serbian politics.

The opposition group that was prevented from protesting in front of parliament, Against the Dictatorship, said the attack against them before the ceremony resembles the time when former Serbian autocrat Slobodan Milosevic ruled the country in the 1990s.

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Associated Press writer Jovana Gec contributed to this report.