Malaysia's Mahathir Mohamad says the opposition alliance he leads has won enough parliament seats to form the next government but the count is continuing and official results so far do not back his claim.
The Election Commission said some results in Wednesday's general election that had gone "viral" were unofficial and had not been verified.
"Of course, political parties can declare whoever (they believe has won), but ... please wait," commission chairman Mohd Hashim Abdullah told reporters shortly after midnight. "We would like to announce it as quickly as possible as well."
Official results showed that, so far, Mahathir's Pakatan Harapan (Alliance of Hope) had won 52 of parliament's 222 seats and Prime Minister Najib Razak's ruling coalition, Barisan Nasional, had won 42.
A simple majority of 112 seats is required by a party or alliance to rule, a number 92-year-old Mahathir said his Pakatan Harapan believed it had won.
"We believe that from our official counting that they're left far behind. The likelihood is that they will not be forming the government," Mahathir told reporters, referring to the prime minister's coalition.
There was no immediate comment from the ruling coalition.
Early results showed that BN had lost seats in key states that have traditionally been its strongholds, raising the prospect that it could be voted out of power for the first time in the country's six decades of independence from Britain.
BN faced a far greater challenge in this election than ever before amid public anger over the cost of living and a multibillion-dollar scandal that has dogged Najib since 2015.
An election eve opinion poll had suggested that support for BN was slipping and Mahathir's alliance would land the most votes in peninsular Malaysia, home to 80 per cent of the population in this Southeast Asian nation.
However, under Malaysia's electoral system, the party or alliance with the majority of parliament seats wins, and going into the poll most experts had believed that was within the prime minister's reach.
Najib, 64, has been buffeted by the scandal over 1Malaysia Development Berhad (1MDB), a state fund from which billions of dollars were allegedly siphoned off.
US Attorney-General Jeff Sessions has described the scandal at the fund set up by Najib as the worst form of kleptocracy. The US Department of Justice has filed several lawsuits to seize more than $US1.7 billion in assets believed to have been stolen from 1MDB.