Zimbabwe's president Robert Mugabe won a huge majority in the parliament in elections the leader of the main opposition party described as "a sham".

Mugabe's Zanu-PF Party won 142 of the 210 parliament seats the Zimbabwe Electoral Commission said.

The results of the presidential vote are to be announced on Monday, but Zanu-PF spokesman Rugaro Gumbo said they expect a "landslide victory" for the 89-year-old controversial leader.

Earlier, Prime Minister Morgan Tsvangirai, who heads the opposition Movement for Democratic Change (MDC), said ballots were a "huge farce" and should be considered "null and void," alleging violations in the voting process.

Tsvangirai's claim that up to a million people were prevented from voting was backed by an independent poll monitor.

Mugabe's supporters rejected allegations, while two of the main observer groups applauded Zimbabwe for holding free and peaceful elections.

The African Union said due to some "grave" and "serious" concerns it was too early to pronounce the elections fair, but irregularities did not constitute evidence of systematic tampering.

The remarks of the continental body were echoed by Southern African Development Community observer mission.

"The question of fairness is broad and you cannot answer it within one day," said mission head Bernard Membe, who is also Tanzania's foreign minister. "And so be sure that within 30 days, through our main report, the question of fairness may come."

The two bodies praised the country' improvements since the last presidential election, which took place in 2008 was marred by violence.

"The Mission observes generally, that from a historical perspective and in comparison to the 2008 elections, Zimbabwe has made an important transition in the conduct of its elections," the African Union said.

Mugabe, 89, has been ruling over the former southeast African British colony since 1980.