Allies of Indonesia's former president Susilo Bambang Yudhoyono have called on President Joko Widodo to intervene after presidential chief of staff, retired army general Moeldoko, was elected chairman of the opposition Democratic Party.

The move came after a hastily called extraordinary congress the party held in Sumatra on Friday saw Dr Moeldoko given the top post. He was not a member of the party before the meeting.

The result saw him displace Mr Agus Harimurti Yudhoyono, Dr Yudhoyono's son.

Mr Andi Mallarangeng, secretary of the Democratic Party's supreme council, said the party is contesting the result, describing Dr Moeldoko's appointment as an attempted coup of the Democratic Party.

He said the congress was organised by former Democratic Party members, including those sacked by the party.

But former party member Sri Mulyono said the congress was the result of cadres growing increasingly unhappy with political dynasties and cronies who "disrupted meritocracy".

He said many were members who worked hard for the party but did not get decent posts, or were sidelined by the party.

Dr Moeldoko, a Muslim and an ethnic Javanese, the majority group in Indonesia, is now tipped to be the party's front runner for president in 2024.

Under Indonesian election law, a presidential candidate must be backed by a group of political parties that hold a total of 20 per cent of parliamentary seats.

Dr Moeldoko, a four-star general who retired as chief of the Indonesian armed forces (TNI) in July 2015, was once tipped to be Mr Joko's running mate in 2014.

Mr Andi is casting doubt on the legitimacy of the appointment, saying the congress was not attended by the minimum two-thirds of the party's provincial branch heads and half of the more than 500 city and regency heads.

The party's supreme council had not approved the congress as well, said Mr Andi, who is calling on President Joko to make sure the Law and Human Rights Ministry would not certify the result.

In a Friday evening address, Dr Yudhoyono told cadre members to defend the party from being hijacked by others.

"If this is a war... it is a war of necessity. A war to seek justice," the former army general said, stressing that he is convinced that Mr Joko, or Jokowi, as he is popularly known, would act wisely.

Until the congress, Mr Agus was the party's top pick for the presidential election. He had revealed early last month that certain parties were orchestrating an attempt to replace him, a move he saw as a precursor to a presidential bid in 2024 using his party as a vehicle.

He said the party's branch leaders were approached by current and former Democratic Party leaders to oust him as chairman.

Mr Agus accused Dr Moeldoko of being behind the plot, an allegation that the former TNI chief had dismissed.

Meanwhile, Mr Andi is urging President Joko to respond promptly, saying Dr Moeldoko should not capitalise on his political appointment to fuel his personal ambition.

"He is a presidential palace inner circle, meets the president every day... The question is whether he (Dr Moeldoko) is carrying out his official duty or doing it for personal interest," said Mr Andi.