The leader of Poland's ruling Law and Justice (PiS) party, Jaroslaw Kaczynski, has told opposition protesters, including MPs occupying parliament's debating chamber, their actions were illegal and could lead to a “great calamity”.

But the opposition vowed to continue a six-day-old protest bringing to a head tensions that have been building ever since the socially conservative and nationalist-minded PiS came to power with a large majority 14 months ago.

In particular, it is demanding a re-run of a debate and vote on the 2017 budget that was diverted to a side-room of the lower house, the Sejm, last week because of the sit-in, and held without opposition MPs present.

The stand-off was sparked last week by government plans to curb media access to the Sejm, the latest in moves by the PiS government which critics say are part of a policy drift towards authoritarianism. PiS denies there is any such trend.

“We are really acting in a restrained way,” Mr Kaczynski told a news conference, flanked by Prime Minister Beata Szydlo, the speakers of both chambers of parliament and a deputy speaker, against a backdrop of white-and-red Polish flags.

“Opposition lawmakers are bound by the law as all other citizens are ... Blocking, taking away freedom from citizens, not allowing normal movements are all criminal acts and these acts are accepted (by the opposition),” Kaczynski said.

“This is really a road to a great calamity.”

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In Brussels, the vice-president of the European Commission, Frans Timmermans, said the government must stop undermining Poland's constitutional court and said the EU would use all its powers to help protect Polish democracy.

Earlier this year, the Commission, the European Union's executive arm, opened a formal inquiry into the rule of law in Poland, a country previously seen as a model for the transition from communism to democratic rule and a market economy.

Warsaw denies it has undermined the top court and says there is no legal basis in EU treaties for the Commission's inquiry.

Mr Timmermans gave Warsaw two months to respond to its recommendations, which would essentially mean rolling back measures Brussels says are at odds with the EU's democratic values and weaken judicial independence in Poland.

“We will not drop this issue,” Mr Timmermans told a news conference, without elaborating.

If all other member states agreed, Poland could lose its voting rights in the 28-nation EU, but Hungary has said it would veto such a move.

Mr Timmermans also said on Wednesday the position of Poland's constitutional court's head must not be filled until questions about the court's independence were resolved.

A few hours before his comment, Polish President Andrzej Duda, a PiS ally, replaced the outgoing head of the court.

Government spokesman Rafal Bochenek described the latest EU step as unnecessary. He said the government has solved its issues with the constitutional court, citing a new law regulating the court and the swearing in of the new court president chosen by the ruling party.

Fears of a drift towards authoritarianism under PiS have prompted protests in cities over the last year. But although a blockade of the parliament building by demonstrators last Friday drew thousands, they had dwindled to dozens by late Wednesday.

The focus of protests has now switched to the budget debate, which the opposition says was conducted irregularly and is invalid.

“If PiS is not open to concessions ... then I do not see other options than to continue the protest until it is effective,” Ryszard Petru, leader of the liberal Nowoczesna opposition party, told reporters.

The protests have mainly been peaceful, but police have now cordoned parliament off with metal anti-riot barriers.

The PiS came to power promising a return to patriotic and Roman Catholic values in public life and a tougher stance towards the EU and Poland's historical adversary Russia.

The government has placed state media and prosecutors under its direct control and changed legislation determining the functioning of the constitutional court.

Poland's financial markets have shown little reaction to the stand-off, with its currency, bonds and share indices all little changed.