By Tim Cocks
LAGOS (Reuters) - Explosions hit an opposition political rally in southern Nigeria on Tuesday and Islamist insurgents struck in the north, with unrest spreading after a presidential election was postponed.
A police officer was killed and several people wounded when five explosions and a burst of gunfire hit an opposition rally in Okrika in southern Nigeria's oil producing Rivers state, home town of President Goodluck Jonathan's wife Patience.
In the north, suspected Boko Haram Islamist bombers killed at least 10 people in three blasts at a military checkpoint and a suicide bombing at a restaurant.
A presidential election due to take place last Saturday has been postponed for six weeks, raising fears of the spread of violence in a country where voting has often been accompanied by unrest.
Jonathan, a southern Christian from the ruling People's Democratic Party (PDP), is facing a challenge from former military ruler Muhammadu Buhari, a northern Muslim from the opposition All Progressives Congress (APC).
Nigeria has roughly equal numbers of Christians and Muslims, and both major parties are active in both the north and south.
Jonathan's opponents accuse him of doing too little to fight the Boko Haram insurgency in the northeast, where thousands of civilians have been killed. They also say the decision to postpone the election to March 28 could lead to more unrest across the country.
Rivers state's outgoing Governor Rotimi Amaechi, a member of the opposition APC who was not at the rally that was targeted on Tuesday, described the bombing there as intimidation by the ruling party: "They don't want people to come out and vote because they know they'll lose," he said.
Rivers PDP spokesman Emmanuel Okah said by phone that it was a clash between rival "cultists", Nigerian university gangs that combine occult rituals with criminal activities.
The rally was staged by Dakuku Peterside, the APC's candidate to replace Amaechi, who is standing down as governor. A policeman died at hospital of wounds sustained in the blast, Rivers Police Commissioner Dan Bature told Reuters by telephone.
Nigeria has a long history of political thuggery and intimidation that have repeatedly marred its polls since the end of military rule in 1999.
Rivers, where Ameachi defected to the opposition APC in 2013, is considered one of the major flashpoints. Unidentified assailants threw dynamite at opposition coalition offices in the state a month ago.
The insurgency by Islamist group Boko Haram, which wants to establish an Islamic state in northeast Nigeria and rejects democracy as un-Islamic, is also surging ahead of the election.
At least three bomb explosions killed eight people at a military checkpoint in the northeast Nigerian town of Biu on Tuesday, witnesses and a hospital source said.
The military fired back on the attackers and killed 17 insurgents, whose movement is seen as the gravest security threat to Africa's top oil producer and biggest economy.
Biu has been repeatedly attacked by Boko Haram and the use of multiple bombs planned to go off in quick succession is a trademark tactic of the jihadists. A nurse at Biu general hospital said eight bodies had been brought in from the blast. Six people were receiving treatment.
In a separate attack bearing the hallmarks of the group, a suicide bomber blew killed two and wounded 12 in a restaurant in the northeast town of Potiskum.
Former Nigerian President Olusegun Obasanjo announced in a statement on Tuesday that he had quit the ruling PDP that he helped found, a blow to Jonathan, whom he had pilloried for failing to tackle Boko Haram and other security threats or stem rampant corruption.
Jonathan and his team argue that they had until recently had little cooperation from neighbors Cameroon, Chad and Niger, all of which have stepped up military operations this month in response to Boko Haram's growing regional threat.
Tens of thousands of people marched through Niger's capital Niamey on Tuesday to support their military following a series of Boko Haram attacks along the Nigeria border.
Nigeria recaptured two towns on Monday.
Boko Haram was cited as a reason for postponing the election, along with delays to distributing voter cards.
Nigeria's electoral commission (INEC) has distributed 75 percent of voter cards to its 68.8 million voters after handing out more than 8 million cards in the 11 days since it announced the postponement of the election.
(Additional reporting by Emmanuel Okolie in Port Harcourt, Tife Owolabi in Yenagoa, Abdoulaye Massalaki in Niamey, Lanre Ola in Maiduguri and Joe Hemba in Damaturu; Editing by Peter Graff)