Crowds of rioters in the port city of Rotterdam torched cars and threw rocks at police who responded with shots and water canon, as protests against COVID-19 measures turned violent on Friday night.

"We fired warning shots and there were also direct shots fired because the situation was life-threatening," police spokesperson Patricia Wessels told Reuters.

"We know that at least two people were wounded, probably as a result of the warning shots, but we need to investigate the exact causes further," she said.

Some people on social media circulated images of someone they said had been shot by police, but the police said that while they had seen the footage they did not yet know how the man was wounded.

Several hundred people had gathered to voice opposition to government plans to restrict access to indoor venues to people who have a "corona pass", showing they have been vaccinated or have already recovered from an infection.

The pass is also available to people who have not been vaccinated, but have proof of a negative test.

Police issued an emergency ordinance in Rotterdam, shutting down public transportation and ordering people to go home. Water canons were deployed and police on horseback carried out charges to disperse the crowds, police said.

The authorities also called on bystanders and people who recorded images of the riots to send the footage to police for further investigation.

The Netherlands re-imposed some lockdown measures last weekend for an initial three weeks in an effort to slow a resurgence of coronavirus contagion, but daily infections have remained at their highest levels since the start of the pandemic. 

Video posted on social media showed burnt out police cars and rioters throwing fireworks and rocks at police.