ATHENS—Clashes erupted between demonstrators and police in central Athens Thursday as some 40,000 people marched during a nationwide walkout against pension reforms.

A small group of hooded youths broke off from the main crowd and threw Molotov bombs and rocks at police, who responded by firing tear gas and stun grenades.

The crowd quickly dispersed but a short while later a group of some 40 youths threw fire bombs at the nearby offices of the socialist Pasok party, causing minor damage.

Two youths have been detained, police said. There were no immediate reports of injuries.

The rally, one the of the biggest seen in Greece in recent months, was held in opposition to pension reforms demanded by lenders in exchange for further rescue funding. It is the third such walkout since Prime Minister Alexis Tsipras' Syriza party was re-elected in September. Several smaller demonstrations were also held on Thursday in other cities across the country.

The march comes after Greece kicked off negotiations with lenders earlier this week on the first review of its bailout program, worth up to €86 billion ($94 billion).

Lenders are demanding that Greece cut its main pensions in a bid to shore up its creaking social security system. The government has counter-proposed that employers pay higher contributions. Talks between the two sides are expected to last for the next few months.

The 24-hour strike, called by public and private sector workers, grounded dozens of domestic flights as ferries remained docked at ports. Schools, courts and pharmacies shut down for the day and state hospitals operated with skeleton staff levels.

Thousands of police officers stood over the march, which ended in front of parliament, as a police helicopter monitored from above.

Most stores in the city center were shut as self-employed groups joined the strike action, arguing that they will be forced to shut down as a result of the new pension plan amid Greece's ongoing economic slump.

Workers and business groups said that they will continue their protest action until the changes are dropped.

The president of the National Confederation of Hellenic Commerce (ESEE), Vasilis Korkidis, described the reform plan as a "monster" that will shut down businesses that have managed to survive the country's six-year debt crisis.

"After so many years and having lost so much…the result is a big zero," he said.

Write to Stelios Bouras at stelios.bouras@wsj.com

(END) Dow Jones Newswires

February 04, 2016 10:35 ET (15:35 GMT)