ATHENS, Greece -- Tens of thousands joined anti-government protests in Athens on Thursday as Greece was crippled by a general strike against a bailout-related overhaul of the country's ailing pension system.

Police said some 40,000 people joined the demonstrations, which were mostly peaceful before sporadic clashes between anarchist protesters and police outside Parliament and in other parts of the city centre.

Police used tear gas and stun grenades against the dozens of hooded anarchists. Street vendors, tourists, and onlookers ran to safety as the violence broke out, while a journalist was attacked by rioters and taken to hospital but was not in serious condition, police said.

Protests were also held in at least a dozen other Greek cities and towns, where several rallies were joined by protesting farmers driving their tractors.

Unions are angry at pension reforms that are part of Greece's third international bailout. The government, which is led by the left-wing Syriza, is trying to overhaul the pension system by increasing social security contributions to avoid pension cuts, but critics say the reforms will lead many to pay up to three quarters of their income in pension contributions and taxes.

Opposition to the reform has been widespread, uniting a disparate group of professions, including farmers, artists, taxi drivers, lawyers, doctors, vets, engineers and seamen.

Thursday's general strike is the most significant the government of Prime Minister Alexis Tsipras, which is in coalition with the right-wing Independent Greeks, has faced since coming to power a little more than a year ago.

The disruption was wide-ranging. Schools and many private businesses were closed while transport, including flights and ferries, faced delays and cancellation. The strike also stopped the flow of thousands of migrants and refugees travelling from the Greek islands where they make landfall.

State-run hospitals were functioning on emergency staff, while farmers maintained their blockades of highways that have forced motorists into lengthy detours.

The strike comes as international bailout inspectors met Labor Minister George Katrougalos to discuss the pension reforms. The central Athens hotel where the inspectors were staying was heavily guarded by police.

The Greek government has to meet a series of conditions to get money from its third international bailout, which is worth around 80 billion euros ($89 billion). Having cleared the first set of conditions, it is now discussing the next batch of reforms required.

Once cleared, it will be due further funds from the bailout as well as a promise to discuss the size of Greece's debt burden, which despite years of austerity still stands at a staggering 175 per cent of the country's annual GDP.

The pension reform is difficult for Syriza, which, while still in opposition, had led protests against pension reforms. However Tsipras was forced to ditch his earlier stance when faced with the stark choice of signing up to a third bailout or the country having to leave the euro currency.

Syriza's difficulties with the pension reform plan were evident in the party's statement that it was backing Thursday's strike.

Athens pensioner Yannis Kouvalakis said Tsipras' government had "fooled" Greeks by promising to reverse austerity cuts.

"Because they are from the left, what happened? Was the situation saved?" he said. "Things got worse."