Explosions and gunfire rocked Bayero University in the northern city of Kano, and witnesses said they targeted two campus church services -- one outdoors, the other in a building but with the crowd spilling outside.
There was no immediate claim of responsibility, although the attack was similar to others carried out by the Islamist militant group Boko Haram which has targeted Christian church services.
The military, which regularly downplays casualty figures in attacks, put the death toll at seven.
However, an AFP correspondent counted six bullet-riddled bodies near one site, and at least another dozen bodies could be seen on a roadside by the university, although the exact number was unclear.
An emergency services official said 22 people were being treated, mainly for gunshot wounds.
Witnesses said the attackers arrived in a car and on two motorcycles, then opened fire and threw homemade bombs, causing panic. They said worshippers were gunned down as they tried to flee.
"They first attacked the open-air service outside the faculty of medicine," one witness said.
"They threw in explosives and fired shots, causing a stampede among worshippers. They now pursued them, shooting them with guns... They also attacked another service at the sporting complex."
A witness, who said he was at the sporting complex, reported hearing gunshots outside while they were praying.
"Then there was pandemonium," he said, recounting how he saw two men outside shooting indiscriminately.
A crowd of people later gathered at a Kano hospital waiting to hear news about friends or family.
The Vatican condemned what it called "terrorist" attacks on Christians in Kenya and Nigeria and called for restraint to prevent a cycle of violence.
In the Kenyan capital Nairobi, a man on Sunday set off a grenade during a church service, killing one worshipper and also sowing chaos.
In another attack against Christians, in the northeastern Nigerian city of Maiduguri, four people were shot dead as they were leaving a church on Sunday, a Christian organisation said.
One of the victims was believed to be a pastor at the Church of Christ in Nigeria, said Mark Lipdo of the Stefanos Foundation which monitors violence against Christians in Nigeria. Authorities could not be reached for comment.
In recent months a wave of violence has convulsed Nigeria, Africa's most populous nation and largest oil producer, which is roughly divided between a mainly Muslim north and a predominantly Christian south.
Boko Haram's increasingly bloody insurgency has claimed more than 1,000 lives since mid-2009. Police and soldiers have often been the victims, but Christian worshippers have also been targeted.
Boko Haram claimed attacks on January 20 in Kano, the largest city in the north, when coordinated bombings and shootings left at least 185 people dead, the highest toll so far.
On Thursday, bomb attacks at the offices of the ThisDay newspaper in the capital Abuja and the northern city of Kaduna left at least nine people dead.
The group has previously targeted churches, including on Christmas Day when at least 44 people were killed in a bombing at a church outside Abuja.
A bombing on Easter Sunday near a church in Kaduna killed at least 41 people, but Boko Haram is not known to have claimed it.
The group also claimed responsibility for a suicide attack at UN headquarters in Abuja which killed at least 25 people last August.
Boko Haram initially claimed to be fighting for the creation of an Islamic state in Nigeria's north, but its demands and structure have become less clear in recent months.
It is believed to have a number of factions, some with political motives, as well as a hardcore Islamist wing. Criminal groups are also believed to have carried out violence under the guise of Boko Haram.
An attempt at indirect dialogue between the group and the government in March collapsed, with a mediator quitting over media leaks and a spokesman for the Islamists saying they could not trust the government.
President Goodluck Jonathan, during a visit Saturday to the newspaper offices in Abuja hit by Thursday's suicide attack, did not answer directly when asked whether dialogue was necessary to stop the violence.
"You may dialogue, you may not dialogue, depending on the circumstances," Jonathan told reporters.
"But we will exploit every means possible to bring this to an end."