Former Polish Interior Minister Mariusz Kaminski has announced that he is on hunger strike to protest his imprisonment for abuse of power. Kaminski was spared a prison sentence by presidential decree in 2015 when his party was in power, but a change of government saw him arrested this week.

The former minister and his ex-deputy, Maciej Wonsik, were detained on Tuesday after a standoff outside the presidential palace, where they were being sheltered by President Andrzej Duda. Both men were sentenced in December to two years in prison, and warrants were issued for their arrest on Monday.

In a statement read by a former aide outside Prime Minister Donald Tusk’s office on Tuesday, Kaminski described his sentence as “an act of political revenge.”

“As a political prisoner, I started a hunger strike from the first day of my imprisonment,” he declared.

The case against Kaminski and Wonsik began in 2009, when they were accused of allowing agents of the anti-corruption body that they headed to use entrapment in an investigation two years prior. Both men were convicted in 2015 and sentenced to three and a half years in prison, but they were pardoned by Duda, who took office that year.

Kaminski and Wonsik were members of the conservative Law and Justice party (PiS) at the time, while Duda is an independent who was obliged to renounce his PiS party membership upon assuming the presidency. He is still considered a close ally of the conservatives.

Duda’s pardon was subsequently annulled by the country’s supreme court, but upheld by a PiS-controlled tribunal.

Poland’s new government, led by the pro-EU Donald Tusk, has claimed that Duda did not have the right to grant the two MPs amnesty. After winning the general election and ousting PiS last month, the new government reopened the case against the former minister and his deputy.

While Tusk’s Civic Platform party controls parliament, Duda is still serving as president.

“I won’t rest until Minister Mariusz Kaminski and his colleague are free men again, as they should be, until they are released from prison,” Duda said on Wednesday. “I won’t be scared. I will act legally, in accordance with the constitution and the law, as before,” he added.

PiS leader Jaroslaw Kaczynski called for a day of mass protests on Thursday, accusing Tusk’s government of selling the country out to the EU.