By Joseph Sipalan and Rozanna Latiff
KUALA LUMPUR (Reuters) - Campaigning for Malaysia's general election started officially 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.
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 for the May 9 elections.
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 defended his government's economic record despite public anger over rising prices, blamed on a consumption tax he introduced in 2015.
Najib will contest from his home state of Pahang. His Barisan Nasional (BN) alliance and the opposition are vying for 222 parliamentary and 505 state seats.
Mahathir told reporters on Friday he suspected sabotage of a private plane that was to fly him from Kuala Lumpur to Langkawi after the pilot discovered some damage just before take-off.
"I have warned before that there will be attempts to stop candidates from making it to the nomination center, but I didn't think it would happen to me," he told reporters at a news conference in Langkawi.
The government ordered an immediate investigation into the claim. Mahathir did not make a police complaint.
The opposition and critics say they are also faced with an electoral system that favors 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 candidates were 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.
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, claiming they had already appointed foreign observers and independent groups.
International observers from Indonesia, Thailand, the Maldives, East Timor, Cambodia, Kyrgyzstan and Azerbaijan will monitor the elections.
A Wednesday polling day set by the commission was also seen as 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.
A low voter turnout was expected to allow Najib's coalition to retain power.
(Editing by Praveen Menon and Paul Tait)