Nigeria's President Muhammadu Buhari says he has ordered the police and military to be "ruthless" with vote-riggers, following the last-minute postponement of the general election.

Critics said his comments condoned "jungle justice".

The incumbent president also called the electoral commission incompetent and ordered an investigation into why the vote was delayed.

Election officials cited "logistical" reasons for the postponement.

Mr Buhari made the comments at an emergency meeting of his All Progressives Congress party (APC) in the capital, Abuja.

"Anybody who decides to snatch [ballot] boxes or use thugs to disturb it [the vote] - maybe this will be the last unlawful action he will take," he said.

He went on to say that anybody who tried to interfere with the election would do so "at the expense of his life".

'Licence to kill'

Nigeria's main opposition party, the People's Democratic Party (PDP), said it was a "direct call for jungle justice".

"It is indeed a licence to kill, which should not come from any leader of any civilised nation," said PDP party spokesman Kola Ologbondiyan.

Mr Buhari also hit out at the Independent National Electoral Commission (Inec).

In his speech to senior party members about the postponement, Mr Buhari said questions needed to be answered about "why such incompetence manifested itself" and promised an investigation.

Inec postponed the election just over five hours before polling stations were due to open on Saturday morning.

They said this was because of problems with transporting ballot papers to some parts of the country.

The presidential and parliamentary polls will now be held on Saturday 23 February.

Governorship, state assembly and federal area council elections have been rescheduled until Saturday 9 March.

Doubts about the new date

However, election observers have told the BBC's Newsday programme that they were not 100% sure the elections would go ahead on Saturday

Festus Mogae, the former president of Botswana, said he was "apprehensive" that all the work that needs to be done, including auditing ballot boxes, will be done in time.

Another international observer, former Vice-President of The Gambia Fatoumata Tambajang, said "it's in doubt because one has to be realistic given the enormity of the activities that are supposed to be taken care of".

Despite these doubts, election commissioner Festus Okoye told the BBC that the commission had ruled out further delays, insisting "there is no challenge any more".

Under the electoral law, campaigning was halted 24 before the election was due to start and the electoral commission said it would remain suspended until polling day on Saturday.

However, the two main parties have said they intended to resume their campaigns.