The leader of Poland’s new pro-democracy movement said the government has “broken the country” after the president, Andrzej Duda, enacted a measure critics say could open the path to authoritarian rule.



The amendment radically changes how rulings are made by the highest legislative court, the constitutional tribunal, and its signing by Duda follows warnings from the European Union and nationwide street protests.

“This is the end of democracy in Poland. They have broken the country,” Mateusz Kijowski told the Guardian. The figurehead of Komitet Obrony Demokracji – whose demonstrations in the past three weeks have drawn tens of thousands of people on to the streets – called for international condemnation of the conservative nationalist Law and Justice (PiS) government, which was elected in October.

“The United States and Europe need to speak loud and clear in condemnation. They must help us, otherwise Poland will leave the community of democracies,’’ he warned.



After news broke on Monday morning that Duda had signed into law the constitutional tribunal bill, he made a speech on television defending his move.



“I think the solution greatly enhances the role and independence of the constitutional tribunal. I believe this act brings to a close the dispute around the constitutional tribunal,’’ said Duda, a lawyer by profession.



Poland’s crisis began last month when the government appointed five judges to the 15-member tribunal.



Mateusz Kijowski called on the US and Europe to condemn the constitutional changes in Poland. Photograph: Paweł Supernak/EPA

The amendment, passed through parliament and the senate over Christmas, requires the court to adopt rulings by a two-thirds majority with at least 13 judges present. Critics say the move will slow down the court and is intended to force it to include the five new judges, who are seen as close to PiS, or leave it unable to pass rulings.

PiS gained an absolute majority in the 25 October parliamentary elections, five months after Duda, who is also from the party, was elected. It is the first time since the end of communism that Poland has been ruled by a single party.



But critics say PiS has a hardline agenda, similar to that of Hungary’s prime minister, Viktor Orbán. In its first weeks in government, PiS removed the provision of IVF under the national health system. Laws currently in parliament include anti-terror legislation that critics say will curtail freedom of movement and expression.

Opposition politician Barbara Nowacka, the leader of the United Left coalition, said: “This is only the beginning. The next step will be a move to control the media and for the prosecutor general to be appointed by the justice ministry.”



She said international condemnation “will not impress’’ the government: “They will not take any notice, just as Orbán has ignored criticism from the European Union. But we, the opposition and the people of Poland, need the support of the international community to defend democracy.’’



She suggested that one of the most powerful external actors could be Britain, due to the estimated 1.3 million Poles living there who send home valuable remittances to families in Poland.



Nowacka said: ‘’David Cameron wants to limit the rights of Poles to claim benefit. The Polish government will have to listen to those threats and, through them, could perhaps be forced to see reason.’’



Polish newspaper Gazeta Wyborcza quoted US sources saying Barack Obama had objections and had let it be known he would delay meeting Duda. The newspaper also suggested Poland’s hosting of the next Nato summit – planned for July 2016 – was in the balance.

The European commission vice-president, Frans Timmermans, and the European parliament president, Martin Schulz, are among those who have criticised the tribunal change.