Two inspectors have been detained over their conduct at Monday's protest in Katunayake, a town near the capital Colombo that is a major free-trade zone.
"The two senior officers have not been charged yet, but they were arrested in connection with the shooting at demonstrators," police spokesperson Prishantha Jayakody said.
A 22-year-old man was critically wounded at the protest and is on a life-support system in hospital, sources said.
About 12 police were among the 150 people wounded.
The authorities swiftly withdrew the pensions bill and declared a holiday in atunayake to defuse tension after thousands of workers took to the streets.
The government had proposed taking over provident fund contributions of private-sector employees and paying them a smaller pension after retirement.