“Continuing political and social tensions [are a] concern" the United Nations Security Council said on Thursday

Demonstrators blocked the centre of Bissau for much of Friday. There was a heavy police and army presence on the streets but no reports of violence. A second demonstration is set for Saturday, which supporters say will be even bigger.

“We will hold demonstrations until the return of legitimate government,” Ibrahima Sory Diallo, vice president of the opposition Party of Social Renovation (PRS) said.

Guinea Bissau, which experienced a civil war between 1998 and 1999, is going through a new period of political upheaval with many of the president’s supporters defecting earlier this month to a new coalition which then passed a no-confidence motion against the country’s prime minister, Aristides Gomes, a long-time ally of the president. CLICK to read an IRIN interview with Prime Minister Gomes in June 2006.

Reports on 20 March said President Vieira had been given 72 hours to either appoint a new prime minister or dissolve parliament, but did neither. Then on Wednesday, Prime Minister Gomes offered his resignation to President Vieira, but as of Friday evening the president had not announced whether the resignation would be accepted.

The head of the PRS, Kumba Yala, who was the country's president until he was deposed in a coup in 2003, promised last year he would return to power “through elections or by force”.

Analysts are keenly watching the reaction of Guinea Bissau’s army to the current political upheavals. It's loyalties are divided between Yala and the president.

“There is no information the military is going to step in at this stage,” a diplomat told IRIN on Friday evening.

Vieira first seized power in a coup in 1980, then went on to win the country's first multi-party elections in 1994, before being overthrown himself in a coup in 1999 after two years of civil war.

His re-election in 2005 was supposed to seal Guinea-Bissau's return to a stable constitutional democracy.