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

But the main suspect in the case, Slovak businessman Marian Kocner, pleaded not guilty to ordering the assassination of the journalist who had been probing his business activities.

The murder 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 being one of two gunmen who killed 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, a town some 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 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."

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 after agreeing 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 Tomas Szabo at the request of his friend Alena Zsuzsova, who was in turn following orders from mastermind Kocner.

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.

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.