WARSAW, Poland (AP) — Polish blood donors were coming forward in droves on Monday in a bid to save the life of Gdansk's mayor, who needs transfusions after being stabbed in the heart and the abdomen while on stage at a charity event.

Doctors operated for five hours on Mayor Pawel Adamowicz, who was stabbed Sunday by an ex-convict who rushed onto the stage with a knife, carried out the attack and shouted it was political revenge against a political party Adamowicz had belonged to.

Adamowicz grabbed his belly and collapsed in front of the audience at the 27th annual fundraiser organized by the Great Orchestra of Christmas Charity.

Doctors resuscitated Adamowicz on the spot and then transported him to Medical University of Gdansk, where he underwent five hours of surgery.

One of the surgeons, Dr. Tomasz Stefaniak, said Adamowicz was in "very, very serious condition" after he suffered a "serious wound to the heart, a wound to the diaphragm and to the internal organs." He said Adamowicz needed massive blood transfusions.

He said the coming hours would be decisive and asked for thoughts and prayers for the mayor who has served since 1998.

Private TVN24 was showing people standing in line and then donating blood in Gdansk on Monday. Some said they were given time off work to help save Adamowicz. A rally against violence was also planned.

Gdansk Archbishop Leszek Slawoj Glodz, who was at the hospital during the surgery, said he was praying for a "miracle."

After the knife attack, the assailant shouted from the stage that he had been wrongly imprisoned under a previous national government led by Civic Platform, a party to which the mayor formerly belonged. He said his name was Stefan and that "I was jailed but innocent. ... Civic Platform tortured me. That's why Adamowicz just died."

Police said the suspect was a 27-year-old who was recently released from prison where he had served a term for bank robberies. A police spokesman, Mariusz Ciarka, said the attacker appeared to have mental problems and gained access to the area with a media badge. It was unclear how he acquired the credential.

He was arrested and is under investigation.

TVN footage showed Adamowicz on stage with a sparkler in hand telling the audience that it had been a "wonderful day" and then the attacker came toward him. The mayor had been on the streets of his Baltic port city earlier in the day collecting money for the nationwide charity that supports Poland's financially-strapped hospitals.

European Council President Donald Tusk, a former Polish prime minister who co-founded Civil Platform and is from Gdansk, tweeted: "Let's all pray for Mayor Adamowicz. Pawel, we are with you."

The head of the charity, Jerzy Owsiak, is a liberal critic of Poland's current right-wing government. Owsiak and some opposition politicians blamed what they described as an atmosphere of hate under the ruling Law and Justice party for the attack.

Adamowicz, 53, was part of the democratic opposition formed in Gdansk under the leadership of Lech Walesa during the 1980s.

After leaving Civic Platform, he was re-elected to a sixth term as an independent candidate in the fall.

As mayor, he has been a progressive voice, supporting LGBT rights and tolerance for minorities. He marched in last year's gay pride parade, a rare action for a mayor in Poland.

He also showed solidarity with the Jewish community when the city's synagogue had its windows broken last year, strongly denouncing the vandalism.

"Horrified by the brutal attack on Gdansk mayor Pawel Adamowicz," tweeted Frans Timmermans, a Dutch politician and leading European Union official. "Hope and pray he will recover. A great leader of his city and a true humanitarian."

The last attack on a politician in Poland was in 2010 in Lodz. A man shouting that he wanted to kill Law and Justice party leader Jaroslaw Kaczynski fatally shot an aide to one of the party's lawmakers to the European Parliament. A second man was stabbed and injured.

At the time, Law and Justice was in the opposition and Kaczynski blamed the attack on an "atmosphere of hate" under rival party Civic Platform.