A transsexual woman and a gay man have taken their seats in Poland's newly-elected parliament – historic firsts reflecting profound social changes in the traditionally Roman Catholic country.

Anna Grodzka, who was born a man, shook hands with several men and women as she entered the assembly hall. She took a seat next to Robert Biedron, a leading gay rights activist who is the first openly gay person to be elected to the Polish parliament.

Both belong to Palikot's Movement, a progressive party that became the third largest in parliament after elections that took place on 9 October.

Palikot's Movement has vowed to push for liberal causes. It opposes the power of the church in society, promotes gay rights and wants to challenge Poland's almost total ban on abortion.

The session was opened by Jozef Zych, a former parliamentary Speaker who invoked words spoken by the late Polish pope John Paul II and acknowledged the presence of archbishops and other church leaders who observed the ceremonial opening from a balcony.

Zych also remembered the late president Lech Kaczynski and MPs who died with him in a plane crash last year. Jaroslaw Kaczynski, his twin brother, sat with other conservative MPs.

Kaczynski heads the country's largest opposition party, the nationalist Law and Justice party, which is riven by deep divisions after expelling three key leaders who had called for a more democratic leadership style.

The 460 MPs of the lower house, or Sejm, were gathering for the first time since the elections gave the centre-right prime minister, Donald Tusk, a mandate for a second term. The 100-seat Senate will meet for the first time later in the day.

The elections marked the first time since Poland threw off communism 22 years ago that a government has won a second consecutive term – another first that reflects growing stability.

Tusk has remained popular thanks to an image he has cultivated of moderation and because the economy has grown impressively since Poland joined the EU in 2004. It was the only EU country to avoid recession during the global crisis of 2008-2009.

The president, Bronislaw Komorowski, addressed the newly-elected body, urging them to work together to maintain Poland's strong economic performance as Europe faces a new financial crisis.

MPs then rose, one by one, to take their vows.

Tusk is to formally resign with his outgoing government later in the day and will then be tasked by the president with forming a new cabinet. The new government will face a confidence vote in parliament in coming weeks. No date for the vote has been set.

Tusk plans to keep governing with his junior partner of the past four years, the conservative agrarian Polish People's party. He also plans to keep many of his key ministers in their jobs, including the foreign minister, Radek Sikorski, and the finance minister, Jacek Rostowski.