Slovak Prime Minister Robert Fico's condition has stabilised but is still "very serious", his deputy said Thursday, a day after Fico was shot multiple times in an attack the government called "political assault".

The attack, which has been condemned around the world, stoked fears over political polarisation or even further violence just weeks before European parliament elections.

Surgeons spent hours in the operating theatre, battling to save the 59-year-old leader after the shooting, which happened as Fico spoke to members of the public after a meeting.

"During the night doctors managed to stabilise the patient's condition," deputy prime minister Robert Kalinak told reporters gathered outside the hospital where the Slovak premier was being treated.

"Unfortunately, his condition is still very serious as the injuries are complicated," added Kalinak, who is also the defence minister and Fico's close ally hailing from his Smer-SD party.

The director of the Banska Bystrica hospital, where the Slovak premier was transported by helicopter after sustaining gunshot wounds, said Fico underwent a "five-hour surgery carried out by two teams".

"He will stay at the intensive care unit," Miriam Lapunikova said.

Footage of events just after the shooting showed security agents grabbing a wounded Fico from the ground and hustling him into a black car. Other police handcuffed a man on the pavement nearby.

Police detained a suspect at the site of the attack in Handlova, President Zuzana Caputova told reporters.

Kalinak said earlier the attack was "a political assault".

"It's absolutely clear, and we have to react on that."

Fico, whose party won the general election last September, is a four-time prime minister and political veteran accused of swaying his country's foreign policy in favour of the Kremlin.

- Unprecedented attack -

Media reported that the suspected gunman was a 71-year-old writer, but police have not named any suspects.

The alleged suspect's son told Slovak news site aktuality.sk he had "absolutely no idea what father was thinking, what he was planning, why it happened".

Analyst Grigorij Meseznikov told AFP this was the first-ever attack on a government minister in Slovakia.

"I only remember the case of former minister of economy Jan Ducky who was shot dead in 1999," he added. "But he had not been politically active anymore when he was killed."

Political analyst Miroslav Radek said he the attack risked causing "further radicalisation of individuals and politicians in Slovakia".

"I am afraid that this attack may not have been the last," Radek told AFP.

The shooting came just weeks ahead of June's European parliament elections in which far-right parties are expected to make gains.

In the central Slovak city of Levice, where the alleged gunman came from, engineer Jaroslav Pirozak told AFP he was sad for Fico.

"But at the same time, he's the one spreading hate and dividing the society, he's the one sowing hatred," the 34-year-old said.

World leaders immediately condemned the attack, including US President Joe Biden who said he and the first lady "are praying for a swift recovery, and our thoughts are with his family and the people of Slovakia."

- Ukraine weapons -

As well as his current stint as premier, Fico headed the government in 2006-10 and 2012-18.

He was forced to resign in 2018 after an investigative journalist's murder exposed high-level corruption and sparked anti-government sentiment.

But he came back again.

Since returning to office last October, Fico has made a string of remarks that have soured ties between Slovakia and neighbouring Ukraine.

He has questioned Ukraine's sovereignty and called for a compromise with Russia, which invaded in 2022.

After he was elected, Slovakia stopped sending weapons to Ukraine.

He also sparked mass protests with controversial changes, including a media law that critics say will undermine the impartiality of public television and radio.

At a press conference following the shooting, MP Lubos Blaha from Fico's party lashed out against the prime minister's critics.

"You, the liberal media, and progressive politicians are to blame. Robert Fico is fighting for his life because of your hatred," Blaha said.