KUALA LUMPUR/LANGKAWI, Malaysia (Reuters) - Campaigning for Malaysia’s May 9 general election began on Saturday, pitting Prime Minister Najib Razak against his former mentor Mahathir Mohamad in a contest marred by claims of sabotage and a skewed electoral system.

Malaysia's Prime Minister Najib Razak (3rd from R) with other candidates arrive at nomination centre to submit his nomination papers to contest in Pekan, Pahang, Malaysia April 28, 2018. REUTERS/Stringer

Najib leads his undefeated ruling coalition into arguably its toughest election since independence from Britain in 1957. He is grappling with a multi-billion-dollar scandal at a state fund, public anger over living costs and an unprecedented challenge by the 92-year-old Mahathir.

Mahathir, returning to politics after retiring 15 years ago, will stand in the holiday island of Langkawi. Prime minister for 22 years before stepping down in 2003, Mahathir returned to challenge Najib after a billion-dollar scandal at state investment fund 1Malaysia Development Berhad (1MDB).

The fund is being investigated for money-laundering and graft in at least six countries, including the United States, Singapore and Switzerland. Najib denies any wrongdoing in connection with losses at the fund.

He has also defended his government’s economic record despite public discontent over rising prices, blamed on a consumption tax he introduced in 2015.

Najib will contest the election from his home state of Pahang. His Barisan Nasional (BN) alliance and the opposition are vying for 222 parliamentary and 505 state seats.

Election Commission Chairman Mohd Hashim Abdullah told a news conference that 85 percent voter turnout was expected on polling day, despite it being a weekday.

“Don’t worry. We set the date and the turnout target because we expect they will come out,” Mohd Hashim told the news conference, which was broadcast live.

A Wednesday polling day was seen as potentially discouraging millions of Malaysians living abroad from voting. However, outraged Malaysians took to social media to offer funding and other services to help people return home to vote.

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A low voter turnout would potentially help Najib’s coalition retain power.

SUSPECTED SABOTAGE

Mahathir told reporters on Friday he suspected sabotage of a private plane that was to fly him from Kuala Lumpur to Langkawi, where he was to file his candidacy on Saturday, after the pilot discovered some damage to the aircraft just before take-off.

“I have warned before that there will be attempts to stop candidates from making it to the nomination centre, but I didn’t think it would happen to me,” he told a news conference.

Mahathir managed to take another flight later on Friday to Langkawi, where was able to lodge his candidacy. Mahathir did not make a police complaint.

The government ordered an investigation. The chairman of the Civil Aviation Authority, Azharuddin Rahman, said later that the inquiry found no indication of sabotage. He said it was wrong to make such “wild and false” claims or political gain.

The opposition and critics say they face an electoral system that favours the BN coalition, compounded by a redrawing of electoral boundaries that was fast-tracked in parliament in March.

Najib’s government and the Election Commission have denied the accusations.

The nomination of at least two opposition party candidates was rejected by the commission on Saturday. Senior People’s Justice Party leader Tian Chua was rejected due to a previous conviction, while another candidate’s documents were incomplete, Malaysian media reported.

Election chief Mohd Hashim said candidates rejected during nominations could file a court petition to challenge the decision.

Other claims, such as discrepancies in the electoral roll, have also been reported.

The Election Commission rejected a request this week by Malaysia’s national human rights body to monitor the election, declaring that it had already appointed foreign observers and independent groups.

International observers from Indonesia, Thailand, the Maldives, East Timor, Cambodia, Kyrgyzstan, Pakistan, Uzbekistan and Azerbaijan will monitor the elections.