Budapest, Dec 17 (AFP) Protesters threw smoke grenades at police who responded with tear gas in Budapest on Sunday as thousands of people rallied against a new "slave law" passed by the government of conservative Prime Minister Viktor Orban.

More than 15,000 people, according to local press reports, joined the demonstration -- the first rally since Orban returned to power in 2010 to bring together all opposition parties, from greens to the far right, under the same banner.

The protest was called by unions and opposition parties outraged at reforms that hike the annual overtime hours that employers can demand from 250 to 400 hours and allows payment to be delayed by up to three years.

The government says the changes are needed by employers short of manpower and will benefit those wanting to work extra hours.

Sunday's protest ended up at Parliament Square, where protesters chanting "Orban get lost!" have been gathering since the law was adopted on Wednesday.

Protesters led by two opposition lawmakers later marched to Hungary's public television headquarters to read a petition but were refused access.

Protesters then hurled missiles and smoke grenades prompting police to respond with tear gas, as they have done in previous days.

"They don't negotiate with anyone. They just do whatever they want. They steal everything. It's intolerable. It cannot go on," said one protester, Zoli, a transport worker.

Protests in the past week have been the most violent in Hungary for over a decade with dozens arrested and at least 14 police injured.

Other reforms passed by parliament, which is dominated by Orban's ruling party, include a bill paving the way for new "administrative courts" to oversee public administration cases.

The justice minister Laszlo Trocsanyi, a close Orban ally, would oversee the courts, leading some to warn the premier could have near-total political influence over the judicial system.

Anger over the legislation has prompted opposition parties across the spectrum, who accuse Orban and his ruling Fidesz party of steering Hungary toward authoritarianism, to join forces.

Pro-government public and commercial media have portrayed the protesters as anarchists and "mercenaries of George Soros".

The Hungarian-born US billionaire Soros has long been accused by Orban of plotting to destabilise Hungary. (AFP) KUN

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