Thousands of protesters marched against austerity measures in central Athens on Thursday, in the fourth major demonstration this year.

Yelling “Thieves, give back our money,” more than 10,000 public and private sector workers marched through the capital towards the parliament building while hundreds of riot police stood guard nearby.

Another 5,000 demonstrators marched in the northern port city of Thessaloniki.

More than 36 people were reportedly detained in a pre-emptive crackdown by police early Thursday, after protesters two weeks ago firebombed a bank, leaving three employees dead.

Greek workers disrupted sea and rail travel and shut down public services across the country, while shop owners closed up early, before the march got under way.

Hospitals were running on emergency staff and schools closed for the day.

Unlike in previous strikes, most flights operated normally after air traffic controllers opted not to join the walkout. Many small regional airports closed, however, and Greece’s Olympic Air was forced to cancel more than 40 domestic flights.

Hundreds of members of the country’s communist party union, or PAME, took over the labour ministry in central Athens early Thursday.

Greece’s debt crisis and subsequent public spending cuts — which includes slashing salaries and pensions — and tax hikes have sparked protests and riots in Greece.

The austerity measures were necessary for Greece to receive a 110 billion euro (140 billion dollar) bailout package from other EU countries and the International Monetary Fund to prevent bankruptcy.

“How does the government expect the younger generation to find work? The average salary is now 560 euros,” said 31—year—old Eleni Mitsou.

European Union policymakers and investors are closely monitoring public reaction amid concerns that large—scale social unrest could prevent the government from pushing through the tough measures.

Greece’s main unions, which together represent about 2.5 million workers, or around half the nation’s workforce, said they would carry out more strikes in June to protest against a pension reform bill they said would further burden the poor.

The bill, which is to be voted on in Parliament in June, would raise the retirement age and discourage workers from taking their pensions early.

Despite the frequency of the protests, opinion polls showed that while most Greeks agreed the austerity measures were necessary, the majority believe the poor and middle class were shouldering the weight of the measures while the rich evade taxes.