At the weekend, the EU and the Carter Center, led by former US President Jimmy Carter, said the polls had fallen short of international standards.

However, both concluded the 11-15 April vote was a significant step towards democracy.

President Omar al-Bashir is expected to be re-elected, after his two main challengers withdrew, alleging fraud.

The former rebels of the Sudan People's Liberation Movement (SPLM) are equally expected to retain power in the semi-autonomous south.

The SPLM has not yet commented on Mr Yakani's allegations.

'Warning'

He says a group of men in a car offered the election observer a lift, asked him if he worked for Sunde, then blindfolded him and drove him to an unknown location where he was assaulted.

"After he was beaten, he was told: 'The information you are collecting for Sunde in Wau - Don't send them to [southern capital] Juba,'" Mr Yakani told the BBC's Network Africa programme.

He said it was impossible to categorically say the attackers were security agents but they acted as though they were and drove a "nice vehicle".

Sunde deployed around 2,000 observers in Southern Sudan and has previously complained of harassment at some polling stations.

The polls - presidential, parliamentary and regional - were the first multi-party elections since 1986.

The complicated ballot in Africa's largest country was beset by problems and heavily criticised by the Sudanese opposition and local observers.

Observers said the ruling parties in both the south and the north also used their huge advantage in resources to influence the vote.