Chiara de Blasio was arrested about 10:30 p.m. Saturday for unlawful assembly, police said.
The 25-year-old was one of about 100 people who refused to leave the roadway when advised by police.
She was given a desk appearance ticket.
Largely peaceful George Floyd protests around New York City Saturday gave way to scattered clashes between police and protesters later in the evening.
Demonstrators smashed shop windows, threw objects at officers, torched and battered police vehicles and blocked roads.
The mayor blamed much of the violence on a small group of well-organized "anarchists."
Thousands gathered in central London on Sunday to offer support for American demonstrators.
Chanting "No justice! No peace!" and waving placards with the words "How many more?" at Trafalgar Square, the protesters ignored U.K. government rules banning crowds because of the pandemic. Police didn't stop them.