On Tuesday a supporter of the former army chief Gen Sarath Fonseka, the main opposition candidate, was shot dead in the south of the island.

Tear gas

The violence in Polonnaruwa left several people injured.

Police used tear gas to disperse thousands of supporters of President Mahinda Rajapaksa or his main rival, Gen Fonseka.

Ms Jayasena was assaulted as she was leaving the scene of the violence to file her report.

Her audio recorders, digital camera, microphone and jewellery were taken by the assailants, police say.

Ms Jayasena has reported for the BBC Sinhala service for several years from Polonnaruwa, which borders the former eastern war zone, and has worked for many other media outlets in Sri Lanka.

It is the first reported violence against journalists in the run-up to the elections. Sri Lanka is one of the most dangerous countries in the world for media organisations to work in.

'Violations'

Amid the violence, monitoring groups say the camps of both President Rajapaksa and Gen Fonseka are misusing state resources.

"Each and every day election violence is rising and election law violations are on the increase continuously," Rajith Keerthi Tennakoon, from the Campaign for Free and Fair Elections, told the BBC.

The BBC's Charles Haviland in Colombo says the monitors place much greater blame on the president's side which has far more resources at its disposal.

They say that ambassadors posted abroad have returned to campaign for the president; that soldiers have been putting up posters in his support; and private phone companies have sent propaganda text messages from the president on orders from the telecoms regulatory body.

The groups also said they were concerned that displaced Tamil civilians who have recently returned to their homes in the north will not be able to vote.

The BBC tried to contact several government ministers for comment but was unable to get a response.