Campaigning for Malaysia's general election began Saturday with flag-waving supporters hitting the streets, as the scandal-hit government fights to retain power in one of the toughest poll battles since independence six decades ago.

Prime Minister Najib Razak faces a stern test at next month's vote due to the controversy surrounding sovereign wealth fund 1MDB and a challenge from his former mentor and veteran ex-leader, Mahathir Mohamad.

Mahathir, a strongman who led Malaysia for over two decades, has shaken up the race with a surprise comeback at the age of 92 as the prime ministerial candidate in an opposition alliance packed with parties he crushed while in power.

But Najib's Barisan Nasional (BN) coalition, which has been in power since independence from Britain in 1957, is tipped to win due to splits within the opposition, and what critics warn will be widespread cheating at the dirtiest election in Malaysian history.

In the unlikely event that Mahathir wins the May 9 poll, he will become the world's oldest prime minister.

There was a festive atmosphere around the country Saturday as candidates formally registered to stand in the election, with flag-waving supporters from different parties parading through the streets.

Once the election commission confirmed their candidacies, the 11-day campaign formally got under way, with 222 parliamentary seats up for grabs.

Najib, in dark blue traditional Malay dress, registered in his constituency of Pekan, in Pahang state, which he has held for over four decades, as his supporters gathered, singing the national anthem and waving flags.

‘He has developed our country, developed the nation, race and country,’ supporter Azizah Abdul Aziz told AFP.

- Holiday island battle -

On the holiday island of Langkawi, Mahathir arrived at a local government office to register, wearing bright red Malay dress and a traditional black cap.

Outside about 5,000 backers of the opposition coalition he is heading, Pact of Hope, waved flags and chanted.

‘Mahathir can help reboot this country,’ supporter Aziah Mohamad Nasir told AFP. ‘Prime Minister Najib has destroyed justice, there is no rule of law.’

Not all candidates registered successfully. Tian Chua, a leading member of opposition group the People's Justice Party, was disqualified from contesting his seat due to a fine imposed by a court for insulting a police officer.

Mahathir was spurred out of retirement to take on Najib -- who heads the United Malays National Organisation (UMNO), the main party in the ruling coalition -- as allegations mounted about the 1MDB scandal.

Billions of dollars were allegedly stolen from 1MDB, which was set up and overseen by Najib, in a campaign of fraud and money-laundering, with American authorities claiming it was used to buy everything from high-end real estate to artwork in the United States.

Najib and the fund deny any wrongdoing, but the controversy has only added to BN's problems after years of falling popularity driven by official corruption, divisive racial politics in the multi-ethnic country and anger at rising living costs.

The 64-year-old leader is under pressure to score an emphatic win after BN lost the popular vote for the first time at the 2013 election.

Najib, who has been premier since 2009, has weathered the 1MDB scandal by sacking critics in government and launching a crackdown that has seen opponents arrested on various charges.

In recent weeks, BN has taken steps that critics liken to gerrymandering -- such as redrawing electoral boundaries in a way the opposition says will tilt the race in the government's favour.

Ibrahim Suffian, head of independent polling firm Merdeka Centre, told AFP that BN was likely to win but signalled it could be a tough fight.

‘There are a few states where the opposition does have a strong presence, so it makes for some uncertainty,’ he said.