Police shot rifles and fired tear gas at protesters who were demonstrating against toll roads on Saturday, and witnesses said at least one person was seriously wounded by gunfire.
Police officers also used batons to assault an Associated Press photographer who was covering the event. His camera equipment was taken before he was thrown into the back of a police truck.
The photographer was released several hours later without charge, and was treated for blunt-force trauma injuries to his neck, shoulder and knee.
Saturday's protest focused on anger over the state government allowing a private company that built a major road in Lagos to collect tolls there.
Lagos State police spokesman Samuel Jinadu told The Associated Press that 18 protesters were arrested.
"Initially, the protest was peaceful, but it later went bad and the police had to make arrests," Jinadu said.
Witnesses said at least one person was shot at the demonstration, but Jinadu said he could not immediately comment on that report.