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.