Opposition supporters pray during a campaign event in Kota Bahru on Friday.

Opposition supporters pray during a campaign event in Kota Bahru on Friday. Photo: Reuters

For days mysterious charter flights have been criss-crossing the South China Sea, transporting tens of thousands of people to vote in Malaysia's tightly-fought election on Sunday.

On arrival in Kuala Lumpur they have been transported by government agencies and accommodated at immigration and army deports, opposition parties claim.

The flights in question were organised and paid for by friends of Barisan Nasional. 

Leaked emails show that at least 40,000 people have been flown from the Borneo states of Sabah and Sarawak to peninsular Malaysia since April 25.

Opposition leader Anwar Ibrahim said the "timing and sheer size" of the arrivals indicated the people had been transported to vote for the ruling Barisan Nasional coalition parties, which are facing their toughest electoral challenge since Malaysia gained independence from Britain more than half a century ago.

But Barisan's secretary-general, Tengku Adnan Tengku Mansor, hit back, saying the transportation of voters was normal electoral practice. He accused the opposition of trying to cast doubt on the fairness of the election as an excuse for its likely defeat.

"The flights in question were organised and paid for by friends of Barisan Nasional," he said. "They brought registered voters to their home districts so that they may vote in the upcoming election."

Tengku Adnan claimed the opposition had chartered buses to ferry voters back from Singapore and that the pro-electoral reform group Berish had paid to fly in Malaysians from other countries.

Many parts of Sabah and Sarawak are government strongholds, while the Malaysian peninsula is home to several closely contested states, including Selangor, near Kuala Lumpur, which fell to the opposition in 2008.

Sabah is a key entry point for foreigners from the Philippines and Indonesia, who have increased the state's population five-fold since the 1970s.

A Royal Commission is investigating allegations that immigrants in Sabah were given identity cards in exchange for voting for the government in a plan approved by former prime minister Mahathir Mohamad.

The Sarawak Report website published emails relating to the flights that it said were leaked by staff of Air Asia and Malaysian Airlines.

The prime minister's office denied any involvement in the flights, the state news agency Bernama reported.

Deputy Prime Minister Muhyiddin Yassin also denied any attempt to improperly boost the government vote.

"We all know there is the electoral roll and people cannot be sent anywhere," he said.

The latest opinion polling indicates the vote pitting Barisan's powerful and long-ruling Malay elite against the Pakatan Rakyat three-party opposition alliance – backed by aspiring young Malays, ethnic Chinese and Indian minorities – will be the closest in the country's history.

While huge crowds have swamped Mr Anwar during rallies, government parties have campaigned on US$2.6 billion hand-outs to poorer voters. A narrow victory by Barisan could trigger street protests and allegations of cheating.

Campaigning has been marred by hundreds of reports of election violence and widespread vote-buying.

Both prime minister Najib Razak and Mr Anwar have pledged they will respect the election outcome.

About 13.3 million Malaysians are eligible to vote for 222 parliamentary seats and 505 state seats. Voting is not compulsory in Malaysia, but analysts expect a record turn-out.