( CNN) Police in Russia imprisoned at the least 1,018 people in nationwide protests against a proposed government pension overhaul Sunday, a Russian independent monitoring group said.

The crackdown was heaviest in Russia's second largest city, St. Petersburg, according to independent local media and OVD-Info, a nonprofit organization that monitors detentions. The group said at the least 452 people were detained in the city.

Images from the St. Petersburg protests promptly ran viral. A local photographer, David Frenkel, confirmed to CNN that he had captured two particularly arresting images: the detention of a school-age boy and an older pensioner.