Sierra Leone counted votes in a tense presidential election on Monday following violence and the death of an opposition party volunteer at the weekend, as international observers voiced concern about lack of transparency in the tallying of ballots.

Provisional results are expected within 48 hours of Saturday's vote, in which incumbent President Julius Maada Bio ran for a second term against a backdrop of public frustration over growing economic hardship in the West African nation.

The head of the main All People's Congress (APC) opposition party, Samuel Kamara, is seen as Bio's main rival.

Police fired tear gas at supporters gathered at APC headquarters in the capital Freetown on Sunday after the crowd turned rowdy, police said in a statement.

A Reuters reporter inside the building found a woman without a pulse lying in a pool of blood under a window with a fist-sized hole in it.

The police did not comment on what happened to the woman. An APC spokesperson said on Monday that she was a party volunteer and that she had died.

The European Union's election observation mission meanwhile said it was concerned about the "highly polarised political environment", and called for transparent vote-counting to build trust in the election process.

The Carter Centre, a U.S.-based election monitoring group, flagged reports "indicating a lack of transparency during parts of the tabulation process".

Sierra Leone's electoral commission said it would respond later.

Many Sierra Leoneans fear more unrest could occur as results are announced, particularly if none of the 13 candidates secures 55% of the votes cast, a situation that would trigger a runoff between the top two.

Schools, offices and most shops were closed in Freetown on Monday. Security forces had cordoned off the APC offices and surrounding areas.

Sierra Leone has been on tenterhooks since unusually violent protests broke out last year over rising prices. Bio and Kamara reported small-scale attacks on their supporters before the election.

Both sides have called for calm. But Kamara questioned the independence of election officials before the poll, expressing concerns about the possibility of vote-rigging.

"Rest assured that I and the APC party would not and shall not accept any skewed, manipulated and unverified results," he said in a statement on Sunday. (Additional reporting by Umaru Fofana Writing by Sofia Christensen, Editing by William Maclean)