Nigerians have returned to the polls, amid pockets of violence, to elect state governors and assemblies in elections, in which the opposition hopes to make gains following its victory that unseated President Goodluck Jonathan two weeks ago.

Tensions are high on Saturday in the south where close contests are expected in Lagos, Nigeria's financial heart and wealthiest state, as well as in the oil-rich southern state of Rivers.

Voters are electing 29 governors and other state representatives from all 36 states.

Al Jazeera's Yvonne Ndege, who is reporting from Rivers capital Port Harcourt, said one police officer was confirmed dead in the state.

Our correspondent also received "unconfirmed reports" that three other people were killed across the southern state, including one who was beheaded and another one shot to death.

"The voting is fraught with major problems," she said, adding that at least two local journalists were reportedly attacked. She also reported incidents of ballot box snatching by armed men "in hundreds of polling stations". 

"In some places, no materials were delivered at all," Ndege said.

Earlier on Saturday, police fired live amunition to control a crowd in Port Harcourt. Voters were angrered because voting materials had not arrived.

At the polling station, where Al Jazeera's Ndege was reporting, voters have been lining up but the voting has not started. There were also reports that voting equipment have not arrived in several polling stations. 

Meanwhile, witnesses told the Associated Press news agency that agitators set a polling station ablaze in Rivers state.

Rivers is seen as a key prize for both Jonathan's ruling Peoples Democratic Party and the opposition All Progressive Congress (APC).

The opposition APC holds 14 states and Jonathan's party 21.

During the March 28 elections, Jonathan lost nationally to the APC's Muhammadu Buhari, who is set to take over as president on May 29.

Tensions have been exacerbated by strained relations between the outgoing Rivers governor Rotimi Amaechi, an APC member, and Jonathan.

In the Rumuola area of the city, about 50 members of APC, which controls the state, staged a protest alleging fraud on the part of the PDP.

"We are protesting that there should be no election in this area because they (the PDP) have changed the ballot papers and results sheets with fake ones," protest leader Kingsley Wuolodi told AFP.

"This is the plot of the PDP to rig the election, so we are saying no election in this place."

The APC claimed the PDP rigged the results of the presidential vote, which saw the state vote nearly 95 percent in favour of President Goodluck Jonathan.

In the city of Lagos, Igbo people have been agitated since the traditional Yoruba leader threatened to kill them if they do not vote for his opposition gubernatorial candidate.

In the northern state of Kano, Al Jazeera's Ahmed Idris, reported that voting was going "smoothly".

"Generally things are peaceful here except for issues with voting accreditation," he said.

Source: Al Jazeera and agencies