One of four suspects charged in the 2018 murder of Slovak investigative journalist Jan Kuciak pleaded guilty in dramatic court testimony on Monday in a case that has rocked the nation.

But the main suspect, Slovak businessman Marian Kocner who is accused of ordering the assassination, refused to comment about his murder indictment, citing "objections" to the contents.

According to the indictment, Kocner decided "to get rid of Jan Kuciak physically and thus prevent further disclosure of his (Kocner's) activities" after failing to find "any dirt" to discredit the journalist.

Kocner had previously pleaded not guilty to ordering the hit on Kuciak who had been probing his business activities.

The shooting triggered mass protests that toppled then-premier Robert Fico and has become a lightning rod for outrage against high-level corruption in the country.

Former professional soldier Miroslav Marcek admitted to gunning down Kuciak and fiancee Martina Kusnirova at their home near the capital Bratislava in a gangland-style hit in February 2018.

"I am guilty," Marcek, 37, told the jury at the Special Criminal Court in the western city of Pezinok, 20 kilometres (12 miles) outside the capital Bratislava.

"I knocked on the door, Mr. Kuciak opened, I shot him in the chest," Marcek told the court.

"Unfortunately, I saw that there was one more person, she ran into the kitchen and I shot her there," he added, referring to Kusnirova, before apologising to the victim's families.

Slovak businessman Marian Kocner denies ordering the killing of investigative journalist Jan Kuciak
Slovak businessman Marian Kocner denies ordering the killing of investigative journalist Jan Kuciak AFP / VLADIMIR SIMICEK

"Seeing them on TV and seeing their pain made me tell what happened," he said. "I'm sorry, but I can't undo it."

Tomas Szabo, another suspect charged in the murder, denied any wrongdoing.

"When Marcek came back to the car he said he found two dead bodies," Szabo said, adding that Marcek, who is his cousin, rejected his suggestion to report the bodies to the police.

"I have nothing to do with the murders," Szabo told the court before refusing to submit to questioning.

Masked, armed police officers provided tight security at the court building where the parents of both Kuciak and Kusnirova attended the trial.

"If convicted, the accused face a minimum 25 years to life imprisonment," Jana Tokolyova, a spokeswoman for the special prosecutor's office, told AFP.

Last month, Zoltan Andrusko, one of the accused who was an intermediary in the murder plot, was sentenced to 15 years in jail on a plea bargain. He will be testifying at this trial.

Prosecutors argue that Andrusko served as a go-between in the murder, hiring gunmen Marcek and his cousin Szabo at the request of his friend Alena Zsuzsova, who was in turn following orders from mastermind Kocner.

Zsuzsova denied arranging the hit with Ardrusko as she testified before the court on Monday, insisting that she had "not ordered any murder in my life" and called any such allegations "an absolute lie".

The protests triggered by the murder that forced populist Fico to quit as prime minister, paved the way for the election of liberal anti-corruption activist Zuzana Caputova as president last March.