WARSAW, Dec 11 (Reuters) - Poland’s media regulator has slapped a 1.5 million zloty ($415,000) fine on leading news broadcaster TVN24 over its coverage of protests in parliament last year, prompting accusations that it aims to muzzle free media.

Critics said the KRRiT media watchdog had punished the U.S.-owned broadcaster in an effort to mute its criticism of the ruling Law and Justice (PiS) party amid speculation the Polish state may take over TVN24.

Earlier this year Polish tax authorities demanded that TVN, TVN24’s parent company, pay an allegedly unpaid 110 million zloty tax, claiming irregularities in a transaction when the group took over a rival in 2012. TVN is challenging the order.

“After a proceeding regarding the method of coverage of events in parliament on Dec. 16-18 (last year), KRRiT concluded that TVN SA, broadcaster of the TVN24 programme, infringed ... a law by promoting illegal activities and encouraging behaviour that threatens security,” KRRiT said in a statement on Monday.

The media regulator did not spell out precisely how TVN24, the most widely watched independent broadcaster in Poland, had allegedly infringed the law in covering the protests.

TVN24, in a statement issued by its press office, said it strongly disagreed with the regulator’s decision and would appeal against it in Polish courts.

“It is very disturbing that for reporting these types of events - the parliamentary crisis and society’s response to it - one of the TV (stations) faces negative consequences,” said Helsinki Foundation for Human Rights lawyer Dorota Glowacka.

“It may be perceived as a certain warning signal for other media that would like to report independently some events that are uncomfortable from the point of view of the ruling party.”

Polish media extensively covered last year’s protests in parliament including a blockade of the rostrum by opposition leaders after PiS imposed curbs on media access to the legislature.

Since winning election in late 2015, the PiS has been increasingly at loggerheads with the European Union over immigration quotas for EU member states and its placing of courts and media under increased state control.