JAKARTA (THE JAKARTA POST/ASIA NEWS NETWORK) - A major street protest is expected to hit Jakarta on Friday (May 24) as supporters of losing presidential candidate Prabowo Subianto are set to join him in filing a lawsuit to challenge the election result at the Constitutional Court.

The rally is expected to be the peak of a string of protests and deadly riots that have gripped the Indonesian capital ever since the General Elections Commission (KPU) announced the victory of incumbent President Joko "Jokowi" Widodo.

The National Police have warned that the street protests might present a major threat to the capital's security, after discovering that some of the suspected rioters arrested over the past two days may be linked with terrorist organisations.

Two among hundreds of people detained by the police for rioting since Tuesday were found to be affiliated with the Islamic State in Iraq and Syria (ISIS) terror group and had intended to launch a terror attack amid the protests, National Police spokesman M. Iqbal said on Thursday.

The police, he said, had also identified another three suspected rioters in possession of firearms, including short and long rifles, who had intended to cause riots by carrying out shootings and making it seem as if the shots were fired by security personnel to turn the public against the authorities.

"These are (two) different groups," Inspector-General Iqbal said, "There are many groups that have the intention of taking advantage of the demonstrations, that's why we have made calls since the very start for people not to participate (in the rallies)."

The two alleged ISIS supporters, who came from outside Jakarta, were allegedly linked to the hard-line Islamic Reformist Movement (Garis), which in the past had expressed support for the ISIS movement and sent its members to fight in Syria, Mr Iqbal said.

"From the testimony of the two arrested suspects, they confirmed their intention to carry out jihad during the protests on (May) 21 and 22," he said.

The peaceful rally on Tuesday afternoon in front of the Elections Supervisory Agency (Bawaslu) headquarters in Jl MH Thamrin in Central Jakarta turned into violence in the evening as dozens tried to break into the building.

The crowd grew larger within hours, with protesters throwing stones, firecrackers and Molotov cocktails at the police, causing the security officers to fire tear gas and water canons.

Groups of rioters were scattered on the street and created unrest in Tanah Abang and Sabang in Central Jakarta and Slipi in West Jakarta until Wednesday night.

"The protesters were calm, but suddenly around 300 rioters came and went on a rampage, launching attacks and damaging facilities," Mr Iqbal said, "We were able to get things under control by 1.25am (on Thursday)."

A total of 185 suspected rioters identified to have been involved in Wednesday night's riots have been detained, adding to the 257 people who had been arrested during riots in different locations since early on Tuesday.

The police said seven had died in the riots, while Jakarta Governor Anies Baswedan reported eight. Nine police officers have suffered injuries.

Mr Prabowo and his campaign team have denied any involvement or responsibility in the street protests and riots, calling them "the actions of provocateurs".

The Jakarta Police, however, confiscated an ambulance emblazoned with the logo of the Gerindra Party, of which Mr Prabowo is chairman, carrying rocks, money and envelopes suspected to have supported rioters.

Three people have been named suspects in connection with the case, all of whom worked for the Gerindra Party branch in Tasikmalaya, West Java, Jakarta Police spokesman Argo Yuwono said.

"They had been ordered by the head of the party's branch (in Tasikmalaya) to head for Jakarta," Mr Argo told reporters on Thursday.

Mr Prabowo's campaign team spokesman Dahnil Anzar Simanjuntak insisted that they were innocent.

He was also not sure that Mr Prabowo would join protesters on Friday.

"We have nothing to do with the rally. The team never initiated a rally," Mr Dahnil said.

An invitation asking the people to come and gather in Jakarta after Friday prayers to support Mr Prabowo went viral on social media.

The invitation mentioned that Mr Bernard Abdul Jabbar, secretary-general of a group called the 212 Alumni, would also participate and lead the rally.

However, Mr Bernard claimed the invitation was fake.

"That's a hoax. There will be no protest," he said.