The first of 19 Russians arrested during a massive protest rally against President Vladimir Putin's re-election has pleaded guilty to criminal charges and was sentenced to four-and-a-half years in prison Friday.

Human-rights activists said they were surprised at the severity of the sentence handed down to Maxim Luzyanin, a 36-year-old businessman. He had pleaded guilty to using violence against riot police who broke up the demonstration six months ago , on the eve of Mr. Putin's inauguration to a third term as president.

Nearly 20 other activists are facing criminal charges arising from the protest in central Moscow's Bolotnaya Square — one in a series of anti-Putin demonstrations since December. The massive rally broke up into clashes between police and demonstrators, with injuries on both sides.

The leader of the Moscow Helsinki Group, veteran human-rights campaigner Lyudmila Alexeyeva, said of Friday's verdict, “This is just the beginning” of what she described as “an offensive against leaders of the opposition.”

Activists have accused police of provoking violence at the rally in May. Alexeyeva noted, however, that only demonstrators were detained or charged in connection with the incident.

Rights activists said they were shocked by the outcome of Luzyanin's case and told Western reporters the court action would inhibit any further guilty pleas, and probably would lead other defendants to draw out court proceedings for as long a time as possible.

Critics have accused President Putin of launching a mass crackdown on dissent.

The Kremlin has maintained that it is operating within the law and is merely taking action against violent, unsanctioned protests in an attempt to strengthen security and keep the public safe.