Marinika Tepic
Serbian opposition leader Marinika Tepic AFP

Looking frail but determined, Serbian opposition leader Marinika Tepic fixes her makeshift bed -- a leather sofa inside a parliament building, where she has been on hunger strike since Monday.

The 49-year-old is protesting what she says was "electoral fraud" during parliamentary and local elections held in Serbia on December 17, and is demanding the results be annulled.

She is not alone. Tepic is one of seven members of the main opposition camp, united under the banner "Serbia Against Violence", to have gone on hunger strike over the results.

Serbian President Aleksandar Vucic's party said it secured a commanding victory during the ballot.

But a team of international observers -- including representatives from the Organization for Security and Co-operation in Europe (OSCE) -- denounced several "irregularities", including "vote buying" and "ballot box stuffing".

Since Saturday, Tepic has been receiving intravenous liquids daily, but refuses to eat food.

"They (the doctors) are trying to maintain this condition as long as possible, because I have no intention to give up until fake elections are annulled, until they admit to electoral fraud, and until the will of the people is defended," Tepic told AFP in an interview.

"This simply needs to be done in order to alert both domestic and international audiences," she added.

Several Western countries have expressed concern over the reported irregularities.

Germany labelled the reported allegations "unacceptable" for a country hoping to join the European Union.

The United States called on Belgrade to address the "concerns" of the election monitors, while the EU said Serbia's "electoral process requires tangible improvement and further reform".

On Saturday, Serbian prosecutors asked the police to probe allegations of fraud.

Serbian police said on Sunday that a total of 344 complaints had been lodged on election day -- less than during the last vote -- and that prosecutors had found elements of "criminal acts" in 18 cases.

International observers have underlined allegations of "voters living abroad being organised and bussed by the ruling party to cast the ballots for local elections in Belgrade".

Tepic has accused Vucic of overseeing the scheme.

"I think that (Serbia) is the only country in the world with a phenomenon of electoral migrants," Tepic said.

Vucic denied the accusations -- in particular addressing the issue of voters who came from neighbouring Bosnia to vote.

"We will defend the electoral will of the people and there is no doubt about that," he said.

The president also urged the opposition MPs to find other ways to protest.

"I would like to ask all those who are on hunger strike not to continue it ... They can organise protests every day. I'm used to protests," Vucic said during a televised address to the nation on Sunday.

Evoking a former head of his party, Tomislav Nikolic, who went on a hunger strike in 2011 demanding parliamentary elections, Vucic said his former party colleague had "achieved nothing but endangering his health".

Tepic however said she was determined to continue.

"It's been a while since I listened to what Aleksandar Vucic says, or at least took it seriously. Because when he opens his mouth, he breaks the law, deceives, lies, buys time and manipulates. And that is how we live for the past 10 years," she said.

Tepic says she is very much aware of the consequences of her actions.

"I try not to think about that (death). I don't see this as a sacrifice. I see this as a fight, and a way that keeps me alive," he said.

"When they ask me if I'm under stress - no. I'd feel more stressed if I did not do this."