HARARE (Sapa-AFP) - Diplomatic moves to end Zimbabwe's post-election crisis intensified on Monday as the opposition mulled whether to contest a presidential run-off after a first round win over veteran leader Robert Mugabe.

Following repeated calls from Western powers for it to show more leadership, diplomats said the African Union dispatched the chairman of its executive arm for talks with Mugabe, who has so far rejected any notion of standing aside despite losing to opposition leader Morgan Tsvangirai in a March 29 election.

While officials in Harare were tight-lipped about the visit by AU commission chief Jean Ping, a senior diplomat based at the AU's headquarters in Addis Ababa said a "very constructive" meeting had taken place with Mugabe.

"Ping and his delegation were received by Mugabe. The discussions were very constructive and provided an opportunity to review all aspects of the situation," the diplomat said on condition of anonymity.

Ping travelled to Harare with AU political affairs commissioner Julia Dolly Joiner and peace and security commissioner Ramtane Lamamra, the source added.

He "also had a working meeting with the chairman of Zimbabwe's electoral commission during which the two men reviewed the entire electoral process from the start," the diplomat said.

"The meeting allowed us to look at all the scenarios for the coming weeks, notably what is being done to ensure a satisfactory second round in the coming weeks," he added.

The talks come as Tsvangirai and his Movement for Democratic Change (MDC) pondered whether Tsvangirai, who fell just short of the overall majority needed to topple Mugabe in the first round, would run in the second round.

Tsvangirai has previously said he sees no need for a second round, convinced he won more than 50 percent in polling on March 29.

However official results released on Friday, nearly five weeks after polling day, gave him only 47.9 percent while Mugabe was said to have won 43.2 percent.

While maintaining the Zimbabwe Electoral Commission (ZEC) is biased in favour of Mugabe, the MDC is also aware any boycott would hand the 84-year-old -- who has ruled the ex-British colony since independence in 1980 -- another six-year term.

"We are saying as far as we are concerned we won and a run-off is not necessary," MDC spokesman Nelson Chamisa told AFP.

"But in the unlikely event that ZEC convinces us that a run-off is necessary, any time any hour we will beat Mugabe hands down."

The run-off should in theory be held within three weeks of the declaration of results but the commission has still to set a date.

Its secretary, Dominico Chidhakuza, played down the prospects of an imminent announcement of a date for the run-off, saying the commission had yet to discuss the issue.

"We are yet to meet. I can't give a date right now because there are issues we are still to discuss," Chidhakuza told AFP.

Many observers believe that Mugabe and his ZANU-PF party, still reeling from their loss of control in parliament in simultaneous legislative elections, are playing for time.

Mugabe's former information minister Jonathan Moyo, now an independent lawmaker, said the authorities were likely to try and delay any run-off by over a month.

"I suppose they are still trying to come to terms with the fact that they have lost parliament, and they stand to lose the presidency and they have not yet done enough to avoid that loss," Moyo told AFP.

"They are mourning, so they want 40 days and 40 nights."

The MDC's fear is that Mugabe and his supporters will use the time to scare voters into backing the president.

Growing incidents of violence, which the opposition says has left 20 supporters dead, have led Western governments to demand observers be sent in to monitor the second round.

Among victims of the violence have been a number of teachers whose union is now warning of strike action from next week.

"What is happening is that you have systematic targeting of teachers, and as result three-quarters of teachers have not gone back to their places of work" since the school term started a week ago, Takavafira Zhou, president of the Progressive Teachers Union of Zimbabwe, told AFP.