Poland politics
A woman holds a banner reading 'Solidarity with Kaminski and Wasik' to protest their arrest in front of the Presidential Palace. AFP

Polish President Andrzej Duda on Wednesday said he was shocked over the arrest of a former interior minister and his aide in a high-profile case that saw one of the detained men launch a hunger strike.

The country has been in political turmoil since a pro-EU coalition took power from right-wing populists of the Law and Justice party (PiS) in December following an October election, with both sides trading accusations of alleged rule of law violations.

Late Tuesday, police arrested prominent PiS figures Mariusz Kaminski and Maciej Wasik at the presidential palace, where the pair had sought refuge following an invitation from the conservative president.

Kaminski, who considers himself a "political prisoner", said Wednesday he would go on a hunger strike to protest his detention.

Both Kaminski and Wasik were elected lawmakers in October's polls, but were stripped of their mandates last month following a court verdict.

"I see my conviction for fighting corruption and the unlawful action to deprive me of my parliamentary mandate as an act of political revenge," Kaminski said in a statement sent to the justice ministry.

But the current government said Kaminski and Wasik were arrested and taken to jail after a court ruling.

"They can't be referred to as political prisoners, it's an exaggeration, also considering the political prisoners detained all over the world," Deputy Justice Minister Maria Ejchart told reporters.

PiS has called on its supporters to join a demonstration on Thursday in front of the parliament building in Warsaw to protest the new administration's decisions, including reforms in the state media.

On Wednesday evening, around 200 people gathered outside the presidential palace in the capital, chanting anti-government slogans and urging the release of "political prisoners", an AFP reporter said.

The two PiS politicians were taken into custody after an appeals court sentenced them to two years in prison for overstepping their authority in a case dating back to 2007.

Kaminski was found guilty along with his then deputy Wasik at the CBA anti-corruption agency for masterminding a fake graft case to discredit a high-level politician.

Speaking to reporters on Wednesday, Duda called for calm in the country, but expressed support for both men, adding that he was "deeply shocked" by the developments.

"I will not rest until Mariusz Kaminski and his associates are free," Duda said.

Both men have denied the charges against them and have evoked a pardon that Duda granted them in 2015. That pardon, however, was later annulled by Poland's Supreme Court.

Legal experts at the time pointed out that the president could not pardon someone who had not yet been definitively convicted.

But as the case gripped the country following the latest court verdict and the arrest warrant, Duda signalled he had no intention to grant them another pardon.

He rejected doubts over the legitimacy of original 2015 pardon, calling it a "textbook case".

As police arrived at the pair's homes on Tuesday to arrest them, they appeared by Duda's side during a ceremony at the presidential palace.

Around 100 PiS supporters -- including the party's powerful chairman Jaroslaw Kaczynski -- gathered late Tuesday in front of the police station where the two men were taken after their arrests.