Biti was arrested minutes after he arrived at Harare airport on Thursday, with police planning to charge him with treason.
 
He could be put to death if found guilty.
 
State-run Zimbabwe Television said in a report that a secret document written by Biti had been unearthed which it claimed showed how teachers employed by the electoral commission had "agreed to overstate the vote" for a payment.
 
Bright Matonga, Zimbabwe's deputy information minister, told Al Jazeera that "Tendai Biti who was on the run decided to come back and hand himself over to the police".
 
"He is facing two counts - one is treason and the second is publishing false statements."
 
Wayne Bvudzijena, a national police spokesman, told the AFP news agency that Biti would be charged "for publishing a document that was explaining a transitional strategy around March 26 which in its case is a treasonous charge".

Bvudzijena said he would also be charged for proclaiming victory for the party in the March 29 joint presidential and parliamentary elections before the official announcement of results.

Major obstacles
  
Tsvangirai officially fell just short of an outright majority over Mugabe in the first round but insists he in fact won more than 50 per cent and is only taking part in a run-off vote under protest.
 
He has faced major obstacles during campaigning, with a series of MDC rallies banned as well as the previous detentions.  
 
The opposition also says that more than 60 of its supporters have been killed since the initial polling as part of an organised campaign of intimidation.
 
Tsvangirai has been detained four times in
the last eight days while campaigning [AFP]
Nqobizitha Mlilo, a senior MDC official, told Al Jazeera that 
"whatever charges may be, the point remains that this is unnecessary harassment on the leadership of the MDC and the people of Zimbabwe".

 

"What the people of Zimbabwe want is change and that change is represented by Mr Tendai Biti, Nelson Chamisa, Roy Bennett - the leadership of the MDC in general - and president Tsvangirai.

 

"This is what the focus should be: to give the people of Zimbabwe a new beginning and a new life."


Biti left the country shortly after he had declared that the MDC had won the general elections, despite official results not yet being released.

 
Haru Mutasa, Al Jazeera's correspondent in Harare, said that the question being raised in the run-up to the run-off vote is how free and fair it will be.
 
She said that the arrest of Biti was a warning to anyone who is trying to be outspoken against the government that they would be severely dealt with.

Call for fair vote
  
Mugabe has banned Western observers from overseeing the polls but he has authorised African monitors to attend the second round, including a team from the 14-nation Southern African Development Community (SADC).

A senior SADC official announced on Thursday that the first batch of 120 observers had begun fanning out across Zimbabwe and said more than 400 should be in place by polling day.
 

Forty African leaders, including former heads of state, business leaders, academics and diplomats, said in an open letter published on Friday that it was "crucial" that the run-off be seen as "free and fair".

 

"It is crucial for the interests of both Zimbabwe and Africa that the upcoming elections are free and fair," they said.

 

Signatories included Archbishop Desmond Tutu and Kofi Annan, the former UN secretary-general.

 

UN action urged

 

Also calling for action from the UN is the US. The White House urged the Security Council on Thursday to take up immediate consideration of the situation in Zimbabwe after reports of "state-sponsored violence" and political arrests there.

 

"We believe the time has come for the United Nations Security Council to take up immediately the issue to prevent further deterioration of the region's humanitarian and security situation," Dana Perino, the White House spokesperson, said.

 

Perino said the US was "deeply troubled" by Biti's arrest.

Source: Al Jazeera and agencies