Malta’s new prime minister, Robert Abela, has a few days to prove whether he will break with “the corrupt legacy” of the past, a son of the murdered journalist Daphne Caruana Galiziahas said.

Abela, an outsider who beat the favourite to become leader of Malta’s ruling Labour party on Sunday, was sworn in as the island’s 14th prime minister on Monday.

He succeeds Joseph Muscat, who is standing down after his top aide became embroiled in the investigation into the killing of Caruana Galizia, who was assassinated in a car bombing in broad daylight in October 2017.

Abela, the son of Malta’s former president George Abela, has promised to strengthen the rule of law and good governance, but is yet to spell out details.

He took the oath of office at a ceremony at the Grandmaster’s Palace in Valletta, led by Malta’s president George Vella, capping a rapid rise for the 42-year old lawyer, who was only elected an MP in 2017.

Andrew Caruana Galizia described Abela as the continuity candidate. “We honestly don’t know what to expect from the new leader,” he said, adding that there were “a few tests, which he will pass or fail very early on”. The tests included whether he would try to revive the political career of the former minister Konrad Mizzi and keep the economy minister, Chris Cardona, in his cabinet.

Daphne Caruana Galizia identified Mizzi, a former tourism minister, as the beneficial owner of a secretive Panama shell company, a link confirmed by an international consortium of investigative journalists including the Guardian. Mizzi has denied any wrongdoing.

Cardona suspended himself from the government in November after police questioned him in connection with the murder case. He was later reinstated.

Protesters in Valletta call for Daphne Caruana Galizia’s killers to be brought to justice. Photograph: STRINGER/AFP via Getty Images

“We will be watching to see what he does and we will probably know within the first few days whether he will really break with Joseph Muscat’s legacy or seeks to protect it,” Caruana Galizia said.

Abela beat his more-fancied rival, the deputy prime minister and health minister, Chris Fearne, in a vote of Labour party members on Sunday. Both promised continuity and neither mentioned the Caruana Galizia case during the campaign.

“Malta is not going through any tragedy, but it is going through a sensitive time from which it will emerge stronger,” Abela told cheering supporters at a sports centre on Sunday.

Sven Giegold, a German Green MEP, who has been a trenchant critic of Malta’s government, said Abela deserved a chance, but that scepticism was appropriate given Abela’s role as a former legal advisor to Muscat. “Abela faces the great challenge of initiating a political restart in Malta. A restart is only possible if he clearly distances himself from the old system of corruption and clientelism,” Giegold said.

“Malta must finally comply with European rules when it comes to anti-money laundering, the independence of the judiciary and banking supervision.”

The Dutch MP Pieter Omtzigt, who has been following the case in the Council of Europe, said he was looking forward to meeting Abela to discuss how the new prime minister would implement reforms to guarantee an independent public prosecutor and root out corruption, after critical evaluations by experts at the Strasbourg body.

“With a new government, there is hope that Malta can bring an end to the damaging recent tensions and start to rebuild its reputation abroad,” Omtzigt said. “The international community is profoundly concerned by Malta’s systemic and structural rule of law deficiencies.”

Muscat congratulated Abela in a tweet and said he was proud to be handing over to him.

Campaigners reacted with fury after Muscat claimed he had “paid the highest price” for Caruana Galizia’s case to be solved.

“Daphne Caruana Galizia paid the highest price, and this case isn’t close to being solved,” tweeted Rebecca Vincent, the UK bureau director of Reporters without Borders. Her post was retweeted by Matthew Caruana Galizia, another of Daphne’s sons, who is at the forefront of the campaign for justice for his mother.

The European parliament accused Muscat, who had won two landslide election victories since 2013, of presiding over “serious and persistent” threats to democracy and rule of law, after a suspect in the murder case made accusations against the prime minister’s chief of staff, Keith Schembri.

Malta’s richest man, Yorgen Fenech, told a court that Schembri had given him updates about the investigation. Fenech has pleaded not guilty to complicity in the murder and all other charges linked to the case.

Police arrested Schembri but later released him without charge. He denies any wrongdoing.