VALLETTA (Reuters) - Activists staged a protest in government headquarters for five hours on Monday to demand the immediate resignation of Maltese Prime Minister Joseph Muscat amid an investigation into the 2017 car bomb killing of journalist Daphne Caruana Galizia.

Muscat is not directly implicated in the case but has said he will resign in mid-January after an election for a new leader of his Labour Party. Testimony by the self-confessed middleman in the murder plot has linked people in the prime minister’s inner circle to attempts at a cover-up.

About 30 activists surprised policemen and soldiers shortly after 7 a.m. (0600 GMT) as they forced their way into the 16th century government building in the capital Valletta from a side entrance, armed with drums, whistles, flags and loud hailers.

They then sat down and blocked the entrance, chanting for Muscat to resign. The prime minister has his office in the building but was not there at the time.

Startled policemen and soldiers responded by escorting journalists out and closing all access to the headquarters, preventing other people, including Daphne Caruana Galizia’s sister Corinne, from joining the demonstration.

Other activists who were planning to take part in the protest then blocked one of the main roads into Valletta.

The political stability of the tiny Mediterranean island has been rocked in recent weeks by the fallout from the murder of the anti-corruption journalist, who was blown up by a car bomb.

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Muscat’s chief of staff Keith Schembri was named by investigators among those alleged to have been involved or having known about the plot. Schembri has resigned and is under investigation. He has denied any wrongdoing.

“Malta has reached rock bottom. This is not just about corruption, but about political murder,” the activists said over a loud hailer during their protest.

They said Muscat’s position was untenable and demanded a comprehensive investigation of all the people named, “not a cover-up”.

The activists made no attempt to go upstairs, where the administrative offices are located, and marched out of the building at midday.