JAKARTA - Indonesia's president Joko Widodo brandished his nationalist and religious credentials on Friday amid reports the Islamic cleric chosen to be his running mate in next year's election was forced on him.

In a last-minute decision, Widodo announced on Thursday Ma'ruf Amin, who heads the board of advisers of the country's biggest mass Muslim organisation, Nahdlatul Ulama (NU), would be his vice presidential candidate for April's poll.

The duo will be challenged by the former general Prabowo Subianto and the private equity tycoon and Jakarta deputy governor Sandiaga Uno.

Wearing a crisp white shirt emblazoned with the slogan "Clean. With the people. Real work.", Widodo told supporters he would "safeguard our national resources".

He cited as evidence policies to nationalise oil and gas assets and seize majority ownership of the huge Grasberg gold and copper mine from the US based Freeport-McMoran.

"It is proof that we are sovereign," he said.

Amin later led the crowd in prayer, asking God to "give us the capability and spirit to safeguard us against forces that try to destroy, to weaken us and our country, or break us apart."

The pair left the stage as Islamic singing, rendered in Arabic, played.

The cleric was then scheduled to deliver a sermon at Friday prayers at Jakarta's biggest mosque.

Prabowo and Uno were to have gone to the mosque before formally registering as candidates with the election commission but changed their plans and were due to pray at another mosque, according to the twitter account of Prabowo's Gerindra Party.

Indonesia politics analyst Marcus Mietzner said Amin's appointment could neuter attacks on Widodo's alleged lack of religiosity and elevate nationalist themes in the election campaign.

"(The opposition) are probably going to shift their focus onto ultra-nationalist themes: sell-out to China, invasion of foreign workers, evil imports, predatory investors," he told Reuters in emailed comments.

"These are classic Prabowo themes anyway, but they will become even more pronounced this time."

Widodo and Prabowo contested the last presidential election, in 2014. Widodo won but his popularity slumped mid-campaign after false reports were spread online that he was a Christian and an ethnic Chinese descendent.

'DEPENDENT'

According to party officials from the president's coalition, Widodo had favoured an alternative to Amin, former constitutional court chief justice Mahfud MD.

In a television interview, Mahfud said he was asked "in some detail" to prepare to be the candidate and was on standby to be anointed on Thursday.

Mahfud said it was Widodo's decision to overlook him.

But, as quoted by the news portal detik.com, NU leader Robikin Emhas said NU leaders, including Amin, told Widodo that they could not support Mahfud.

Mietzner said Widodo had initially discounted NU's opposition to Mahfud.

"The implications are that four years into his presidency, Jokowi is still much more dependent on his supporting parties than he cares to admit. (It's) a demonstration of his continued weakness," referring to Widodo by his nickname.

A leader of NU-aligned National Awakening Party (PKB), Muhaimin Iskander, told reporters on Thursday it was a surprise Amin was selected and that Mahfud was Widodo's initial preference.

"I thought Mahfud was chosen. But it turned out to be Ma'ruf Amin," he said.

Kompas newspaper quoted another PKB leader, Abdul Kadir Karding, as saying the decision to choose Amin happened on Thursday afternoon.