Warsaw (AFP) - President Andrzej Duda on Monday signed into law a controversial bill reforming Poland's constitutional court despite widespread domestic opposition and mounting alarm in the international community.

Thousands of people have taken to the streets over changes pushed through by the governing conservative Law and Justice Party (Pis) that the opposition says threaten judicial independence.

The eastern European powerhouse has been plunged into a political crisis since the PiS won an election in October after eight years in opposition.

Duda's move came four days after the legislation was adopted by parliament, triggering EU warnings that the reforms could undermine rule of law.

"The commission is following the constitutional changes in Poland with concern," a spokeswoman for European Commission chief Jean-Claude Juncker told AFP in Brussels.

The commission is due to discuss the crisis on January 13, after Vice President Frans Timmermans wrote to Poland's foreign and justice ministers last week urging implementation of the reforms be suspended until all questions about their impact "have been fully and properly assessed".

Poland's opposition accuses the PiS of seeking to take control of the court to remove important checks on government power.

But Duda, who is backed by the PiS, remained defiant on Monday.

"After analysis and reflection, I remain clear: I consider that this change will help strengthen the status and role of the constitutional court," he told reporters.

Under the legislation, which takes effect immediately, the court must approve rulings by a two-thirds majority rather than a simple majority as at present, and 13 of its 15 judges must be present for the most contentious cases, up from nine.

It also introduces obligatory waiting periods of three to six months between the time a request for a ruling is made and a verdict, compared with the current two weeks.

- 'Undermining democracy' -

The feud erupted when the party installed five judges of its own choosing to the court, using a controversial procedure to overrule appointments made under the former centrist government to boost the number of conservative voices on the bench.

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Tens of thousands of people took to the streets of Warsaw in mid-December in protest at the government's actions.

The PiS leader, staunch conservative ex-premier Jaroslaw Kaczynski, has said he wants to break up the "band of cronies" he says made up the court, which he accuses of trying to block the ruling party's policies.

Kaczynski is neither premier nor president but is widely thought to call the shots in the party and is the undisputed boss of Poland's populist Catholic right.

He is known as a fan of the political model of Hungarian hardline Prime Minister Viktor Orban, who has cracked down on media freedoms and the justice system in his own country.

The PiS also has plans to overhaul state media including public television and radio into national cultural institutions like the opera or the national museum.

Former president Lech Walesa, who led the Solidarity movement that brought an end to communism in Poland, has accused the new government of acting against the country's interests and undermining democracy and called for a referendum to force early elections.

The office of the UN High Commissioner for Human Rights has likewise expressed "concern" about the reforms.