AMSTERDAM, Jan 2 (Reuters) - Riot police with batons and shields broke up a crowd of several thousand who had gathered in Amsterdam on Sunday to protest against COVID-19 lockdown measures and vaccinations. Public gatherings of more than two people are prohibited under restrictions imposed by the Netherlands in an effort to prevent the Omicron variant of the coronavirus overwhelming an already strained healthcare system. At least 30 people were detained after scuffles, during which four officers were injured, police said in a statement. Amsterdam mayor Femke Halsema had issued an emergency ordinance, empowering police to clear the central Museum Square, after the protesters defied a ban on public gatherings. The protesters, who mostly did not wear masks or abide by social distancing rules, ignored an order not to hold a march and walked along a main thoroughfare, playing music and holding yellow umbrellas in a sign of opposition to government measures. The Netherlands went into a sudden lockdown on Dec. 19, with the government ordering the closure of all but essential stores, as well as restaurants, hairdressers, gyms, museums and other public places until at least Jan. 14. read more Reporting by Hilde Verweij; Writing by Anthony Deutsch Editing by Mark Potter and Alexander Smith