Zimbabwe's defeated presidential candidate Morgan Tsvangirai on Monday prepared to mount a legal and political challenge against the "sham" election that looks set to extend Robert Mugabe's 33-year rule.

Tsvangirai's allies have announced they will launch a constitutional court challenge against the results of Wednesday's election, which handed Mugabe a thumping 61 percent of the vote.

"Our lawyers are very busy at work. We will be lodging the presidential challenge before Friday," Douglas Mwonzora, spokesman for Tsvangirai's Movement for Democratic Change (MDC), told AFP.

The case could delay 89-year-old Mugabe's inauguration for another five-year term.

Once the complaint is lodged, the country's top court has 14 days to reach a decision.

Western nations, including former colonial ruler Britain, voiced serious doubts about the election while the regional SADC bloc said it was "free and peaceful" but stopped short of describing it as fair.

But MDC insiders acknowledge that finding a smoking gun for electoral fraud and navigating the notoriously polarised court system with be a fiendishly difficult.

The ruling ZANU-PF has welcomed the prospect of a court challenge over the vote -- the first since bloody 2008 elections led to the formation of an uneasy power-sharing pact between Mugabe and Tsvangirai.

"What they are doing is a good thing, it's a wise road to take," ZANU-PF spokesman Rugare Gumbo said, expressing confidence the challenge will fail.

Constitutional law expert Lovemore Madhuku said that given the 61-34 percent vote split against Tsvangirai, his chances "are non-existent, completely non-existent".

"It's not a margin that normally gets challenged in court."

It is the third time that Tsvangirai, a 61-year-old former union leader, has tried and failed to unseat Mugabe.

Mwonzora said the MDC is preparing a "dossier of all the rigging that took place and we will put it in the public domain to show the people how the election was stolen."

The MDC has called for post-election an emergency summit of the regional bloc Southern African Development Community (SADC) after the group gave the vote a thumbs-up, but deferred on calling the election "fair".

South African President Jacob Zuma on Sunday extended his "profound congratulations" to Mugabe on his re-election after a "successful" vote while his Namibian counterpart Hifikepunye Pohamba also welcomed the vote.

"The people of Zimbabwe have once again demonstrated their trust and confidence in the Zanu-PF and your party leadership," he wrote in a letter to Mugabe.

The MDC's Mwonzora -- who himself lost his seat -- expressed regret at the verdict of Zimbabwe's neighbours.

"We discovered that the SADC and the African Union equate absence of bloodshed in elections to free and fair elections, which is an incorrect way of looking at things."

The MDC had more support further afield, with the United States and European nations including Britain condemning the election, and Australia even calling for a re-run.

Mugabe and his allies still face a number of Western sanctions.

The Zimbabwe stock exchange plunged 11 percent on Monday, the first day of trading since the results were issued, with foreign-owned stocks registering steep losses.

The MDC also took a hammering in the parliamentary vote, winning just 49 seats out of the 210 up for grabs -- down more than half from its 2008 performance -- against 160 for Mugabe's party.

Analysts say if the MDC is to rebound politically in the face of its defeat, it needs to completely rebuild.

"It needs a comprehensive introspection... just as ZANU-PF did when it was almost on its knees in 2008," said University of Zimbabwe political analyst Eldred Masunungure.

"That may entail leadership review especially at the top," he said.

Mwonzora however dismissed suggestions that it was time Tsvangirai was replaced as party leader.

"We stand solidly behind Morgan Tsvangirai as our president," he said. "The MDC is regrouping, we are gearing for a fresh election whenever it comes.

"If there is anything which has reunited us... (it) is this theft."

Aggrieved MDC parliamentary hopefuls who are alleging fraud are due to have their complaints heard by the electoral court.

These must be heard and settled within six months but will not stop the swearing-in of deputies.