A French policeman was taken into custody for questioning on Thursday over the assault of a teenager last week during protests against labour reforms, police and judicial sources said.
A video of the confrontation caused outrage on social media last week and triggered protests outside police stations in Paris.
The video shows two policemen holding up a 15-year-old boy while another violently punches him, on the sidelines of student protests.
The policeman was interviewed on Thursday morning, the sources said, following the opening of a case by Paris prosecutors.
Both Interior Minister Bernard Cazeneuve and Paris police chief Michel Cadot described the video of the incident as "shocking." "We were protesting, throwing eggs. A policeman... charged at me, hit me to the ground then he told me 'get up, get up' and he punched me... my head was spinning," the boy told French media last week.
The arrest of the policeman on Thursday coincided with a fresh day of protests across the country against the proposed labour reforms.
Last week's demonstrations saw cars torched in the capital and clashes with riot police, who responded with tear gas and made more than 30 arrests.