JAKARTA: Supporters of an unsuccessful presidential candidate clashed with security forces in the Indonesian capital on Wednesday, burning vehicles and throwing rocks at police using tear gas and rubber bullets.

The protesters tried to force their way into the downtown offices of the election supervisory agency late Tuesday and clashes have continued since then. National Police spokesman Dedi Prasetyo said more than 20 suspected provocateurs have been arrested.

Local media reported several deaths among protesters but there has been no official confirmation.

White-robed protesters blocked streets in one central Jakarta neighbourhood and in another, rioters fought running battles with police, throwing rocks and setting fires.

During the night, vehicles and a paramilitary police dormitory were set ablaze as police using tear gas, rubber bullets and water cannon battled protesters who threw molotov cocktails and burning projectiles.

Indonesia's Election Commission on Tuesday said President Joko Widodo had won a second term with 55.5% of the vote in the April 17 election.

Former special forces general Prabowo Subianto has refused to accept the results and declared himself the winner. His campaign plans to challenge the election in the Constitutional Court. They allege massive fraud but have provided no credible evidence.

The government had deployed some 50,000 police and soldiers in Jakarta in anticipation of protests, said Jakarta police spokesman Argo Yuwono. Many residents have left the city and parts of the downtown are closed to traffic with the election supervisory agency and election commission barricaded with razor wire.

In the past week, authorities have arrested three pro-Subianto activists on suspicion of treason, said Prasetyo, including a retired general and former commander of Indonesia's special forces. Police allege there was a plot to seize crucial government buildings in Jakarta.

Subianto and members of his campaign team had said they would mobilize "people power" for days of street protests. The former general has also called on supporters to refrain from violence.

Subianto, who also lost to Widodo in 2014, ran a fear-based campaign, emphasizing what he sees as Indonesia's weakness and the risk of exploitation by foreign powers or disintegration. He aligned himself with hard-line Muslim groups and won massive majorities in conservative provinces such as Aceh, which follows Shariah law, but was defeated by Widodo in the president's populous East Java and Central Java strongholds.

Widodo's campaign highlighted his progress in poverty reduction and improving Indonesia's inadequate infrastructure with new ports, toll roads, airports and mass rapid transit.