THESSALONIKI, Greece — The 75-year-old mayor of Greece’s second-largest city was brutally assaulted by a far-right mob at a public gathering over the weekend, an attack that brought gleeful expressions of support from nationalist groups and heightened concerns about a rise in hate crime in the country.

The mayor, Yiannis Boutaris, who is known for his liberalism and outspoken views against far-right violence and racism, was attacked Saturday afternoon in the northern city of Thessaloniki by about a dozen people during a ceremony commemorating the World War I genocide of Pontic Greeks by Turkish forces.

In video footage of the attack on the mayor, the white-haired Mr. Boutaris is heckled by a crowd of men, some masked and dressed in black. He was thrown to the ground, punched and kicked before being escorted to a car that was vandalized by protesters, and he was briefly hospitalized with head, back and leg injuries.

Nationalist groups, notably the far-right Golden Dawn, have capitalized on growing frustration in Greece. The economy has been hobbled by years of austerity imposed on the debt-racked country by international creditors, and a huge migrant influx has further strained tensions.