At least 56 people have died across the country since the demonstrations began, with about 38 killed nationwide on Tuesday alone, according to Amnesty.
The African Union on Thursday strongly condemned the violence and called on all parties to "privilege dialogue".
Nigeria's army has dubbed as "fake news" reports that soldiers opened fire on demonstrators.
Shops were shuttered and streets empty Thursday as Nigeria's largest city Lagos was locked down under curfew after unrest flared following the shooting of protesters.
Amnesty International said at least 12 people were killed by the Nigerian army and police in a brutal crackdown on protesters Tuesday that drew international condemnation.
