European Council President Donald Tusk called on Poland's political leaders to respect the constitution in a speech in Warsaw on Friday, amid speculation that he may return to domestic politics when his EU term ends in November.

The former Polish prime minister and leader of the opposition Civic Platform party accused political leaders of celebrating the constitution once a year and disrespecting it on every other day, during a speech at Warsaw University to mark the annual constitution day.

"It can't be that those in power once a year celebrate constitution day, but disrespect it on a daily basis," he said.

There is widespread speculation about Tusk's future once he leaves his European post. Poland is due to hold a parliamentary election in the autumn and a presidential election in mid-2020.

Tusk did not announce a return to Polish politics on Friday. He criticised political divisions in his home country, saying: "Today, specifically in our country, politics is taking on the character of a domestic war."

The European Union has begun a legal procedure against Poland over changes to its judicial system by the governing nationalist party that EU officials say undermines the democratic separation of powers and the rule of law.