* Military leant on electoral commission to delay poll
* Critics fear poll could be indefinitely postponed
* Gov't says security legitimate reason to delay
By Tim Cocks
ABUJA, Feb 9 (Reuters) - Nigeria's agreement to delay this week's election on the advice of security forces creates a worrying echo for some of the annulment of 1993's democratic vote by a military government.
The election this Saturday in Africa's biggest economy was set to be the closest fought since the end of military dictatorship in 1999. President Goodluck Jonathan is seeking re-election in a contest with main opposition candidate Muhammadu Buhari, himself a former military ruler.
Britain and the United States have criticised the postponement, termed "a coup against the constitution by the security chiefs" by Lagos-based lawyer Femi Falana.
"As far as the law is concerned, the armed forces have nothing to do with the elections," said Falana, seen as an opposition sympathiser.
Accusations of interference have been dismissed as absurd by the team around President Goodluck Jonathan, who point out that the advice was simply to shift the date to March 28 owing to a worsening Boko Haram Islamist insurgency in the northeast.
However Jonathan's ruling People's Democratic Party (PDP) had previously heaped pressure on the electoral commission INEC to delay the poll, arguing it was not ready because millions of voters had yet to collect their voter cards. INEC insisted the vote would happen on time, until the security forces stepped in.
Observers now fear further election delays or even a cancellation, which would trigger unrest and further rattle worried investors.
"Having used (the security forces) to shift the election, what's to stop them saying there will be no election?" Falana said, drawing a comparison with 1993, when then military ruler Ibrahim Babangida annulled a vote after a candidate he didn't like won it.
Government officials point out that unlike in 1993, Nigeria - Africa's most populous country - faces a raging insurgency.
"This is the first time the country has been under siege like this," government spokesman Mike Omeri said by telephone.
The opposition All Progressives Congress (APC) accuses the PDP of seeking to buy time.
A PDP campaigner in a southern state, who declined to be named for fear of reprisals, said Jonathan's camp was deeply disappointed at his reception at some party rallies. In some he has been booed and heckled and his convoy stoned.
INEC chairman Attahiru Jega, who announced the delay on Saturday night, said he changed his mind after receiving a letter on Feb. 4 from the office of the National Security Advisor.
"The letter stated that security could not be guaranteed during the proposed period in February for the general elections," Jega said.
Jonathan's camp rejects the charge from opposition supporters that the INEC was boxed in in order to frustrate the democratic process.
"We're talking about just a six-week postponement to ensure the election is done credibly. There's no way it can affect the outcome of the election," PDP spokesman Olisa Metuh said by telephone. "It confers no advantage whatsoever on the PDP."
But Buhari said on Sunday that if the real reason for the delay was a worsening insurgency in the northeast, it seemed unlikely that a six-year problem can be defeated in six weeks.
That in turn raises the possibility the government could "postpone, re-postpone and ultimately cancel the elections, under the pretext of a national emergency," said Sebastian Spio-Garbrah of U.S-based consultancy Damina Advisors in a note on Monday.
The latest date an election can take place is 30 days before Jonathan's mandate runs out on May 29th, according the constitution.
The uncertainty has tipped the country's financial markets into turmoil, with the naira sliding to a record low on Monday.
Omeri said the government was confident that with better cooperation with neighbouring Chad, Niger in the fight against Boko Haram, the situation would be more secure in six weeks.
The extra time will also give INEC more time to work out the logistics of the vote in the war-affected areas, he said. (Editing by Sophie Walker)