The boat capsized early on Monday on the Niger River in the state of Kwara close to neighbouring Niger state, police spokesman Okasanmi Ajayi said.
"What we gathered was there was a boat that capsized and about 100 people died," he said.
The victims, including women and children, were returning from an overnight wedding ceremony in the village of Egboti in Niger state, resident Usman Ibrahim said.
It was not immediately clear if there were survivors.
"People in the boat were to take their bikes to their various communities (upon disembarking). The boat was carrying more than 100 people when it sank," he said.
Many of the victims drowned because the accident happened about 3am and not many knew of it until hours later, he said.
"Up till now, some dead bodies are still being searched (for)."
As of Tuesday afternoon, officials and locals were still searching for more bodies in the river, which is one of Nigeria's largest.
Boat accidents are common in many remote communities across Nigeria where locally made vessels are commonly used for transport.
Most accidents are attributed to overloading and the use of poorly maintained boats.
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