It was the latest in a series of attacks blamed on radical Islamists who have vowed to wage a religious war on Nigeria's Christians and drive them from the country's majority-Muslim north.
Several dozen Christians had come together for a meeting in a town hall in Mubi, in Adamawa state, to mark the deaths the day before of several people killed in the town.
Up to four gunmen surrounded the building and opened fire with Kalashnikov rifles, killing up to 20 people and leaving another 15 badly injured.
"We started hearing many gunshots through the windows," said Okey Raymond, 48, who was at the meeting.
"Everyone scampered for safety, but the gunmen chanted: 'God is great God is great' while shooting at us."
No arrests have been made in the attack, and no one has claimed responsibility.
A purported spokesman for Islamist group Boko Haram claimed responsibility for the attack.
Mubi is close to the Cameroon border and is not in an area covered by a state of emergency declared by Goodluck Jonathan, Nigeria's president, following two weeks of sectarian violence.
The country's population of 160 million people is roughly divided between a mainly Muslim north and predominantly Christian south.
While Boko Haram has been blamed for increasingly deadly attacks for months, including an August suicide bombing of UN headquarters in Abuja that killed 25, the violence has taken on a different dimension with recent church attacks.
A wave of Christmas bombings that killed 49 people, most of them outside a Catholic church as services were ending, has provoked outrage in Nigeria and intensified fears of more sectarian clashes.
There have been fears of reprisals from Christians, and Christian leaders have warned they will defend themselves if attacks against them continue.
Boko Haram is a shadowy group believed to have a number of factions with differing aims, including those with political links and a hard-core Islamist faction.
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