ABUJA, Nigeria (AP) – Opposition parties joined forces Wednesday to call for a postponement of the presidential vote scheduled for this weekend, as fighting broke out between troops and suspected Islamists in northern Nigeria.

The gunfire erupted in the town of Kano near an area where the group of suspected Islamists were holed up after they attacked a police station on Tuesday, killing 12 policemen and one civilian.

Residents said they believed the attackers were members of an outlawed, self-styled “Taliban” Islamist movement that has clashed with security forces in recent years. Gunfire could be heard and witnesses said they saw injured troops being rushed from the scene.

Meanwhile, a communique signed by 18 parties came just hours after the country’s electoral commission said it would abide by a Supreme Court ruling that put Vice President Atiku Abubakar back on the ballot for the presidential election. He had been barred from running on allegations of corruption, charges he denies.

The communique also called on Nigerians to “protest in a nonviolent manner” against election results that showed the governing party rolling up huge victories in state governor and legislator races.

The opposition parties, including those of presidential candidates Gen. Muhammadu Buhari and Abubakar, demanded a “level playing field” for all candidates and threatened to boycott Saturday’s vote setting up Nigeria’s first-ever transfer of power from one civilian leader to another.

The parties called on Nigerians to protest “these sham elections so far held, and to resist further elections,” until the electoral structures are amended. They also demanded a new electoral commission.

Government officials weren’t immediately available for comment. The electoral commission said before the opposition parties issued their communique that the presidential vote would take place Saturday.

The opposition’s late-night demands after an hours-long meeting threaten to further endanger the drive to cement civilian rule in Africa’s most-populous nation.

Violence and irregularities marred state elections last weekend. Authorities reported at least 21 people were killed, while many voters waited for hours in front of polling stations that opened late with faulty voter-registration rolls. In some areas, people were seen stuffing ballot boxes.

Mr. Abubakar told AP Television News on Tuesday that his being put back on the ballot was “a victory for democracy.”

“It is a victory for the rule of law, and the protection of the constitution in this country,” he said. “The judiciary has to be commended for its courage, for its defense of our young democracy.”

Police banned political rallies across Nigeria, and security forces were ordered to put down any clashes.

Still, violence hit the northern city of Kano, which has been swept by riots in past years and is under a nighttime curfew after unrest in recent days. Gunmen torched a police station Tuesday and killed 12 policemen and the wife of an officer, said the regional police spokesman, Baba Mohammed.

Residents said they believed the attackers were members of an outlawed Islamist movement that has clashed briefly with security forces in recent years. The attackers told people in the neighborhood that their quarrel was with the government only, but many people fled, residents said.

Some of the attackers were apparently holed up in a building near the police station, with troops nearby. Security forces kept reporters back.

Mr. Abubakar is making a run for the presidency after bolting from the ruling party. Last year, he successfully opposed a drive to amend the constitution and allow a third elected term for President Olusegun Obasanjo. Umaru Yar’Adua, a member of Obasanjo’s party, is seen as the front-runner in the presidential race.

The electoral commission had barred Abubakar based on findings by an executive panel established by Mr. Obasanjo that found Mr. Abubakar stole government funds. Abubakar denies the allegations.

The electoral commission had said earlier it had already printed ballots for 61 million voters that will have to be distributed across an impoverished country larger than France, dotted with far-flung villages. Umeadi insisted, however, that the vote would take place as scheduled.

Since independence from Britain in 1960, most Nigerian elections were scuttled by military coups or annulments. Obasanjo, a former military ruler, was elected in 1999, ending decades of near-constant military rule and coups d’etat that overturned periodic civilian administrations.

Mr. Obasanjo’s 2003 re-election was marred by violence and accusations of widespread rigging.

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Associated Press Writer Salisu Rabiu in Kano contributed to this report.