Haiti's government installed a new prime minister on Tuesday, while officials mourned assassinated President Jovenel Moise and arrested at least three police officers implicated in his killing.

Designated Prime Minister Ariel Henry was sworn in to replace interim Prime Minister Claude Joseph, who assumed leadership of Haiti with the backing of police and the military after the July 7 attack at Mr Moise's private home, which also badly injured his wife and stunned the nation of more than 11 million people.

"The task that awaits us is complex and difficult," Mr Henry said as he called for unity and promised to meet with various sectors in forthcoming days to build consensus. "I'm from a school that believes in dialogue."

He also thanked "sister nations" that he did not identify for their solidarity and expertise in the ongoing investigation as he demanded that all those responsible be brought to justice.

Mr Henry, a neurosurgeon and former Cabinet minister, has promised to form a provisional consensus government to lead Haiti until elections are held. He said he had already met with various unidentified stakeholders as well as people from the civil service and the private sector.

He pledged to re-establish order and security and confidence in the government as well as fight corruption, make Covid-19 vaccines available to all, restart the economy and create a credible and transparent elections system.

Mr Joseph, who spoke before presenting Mr Henry, also urged unity and and wished him courage and determination while warning him: "You inherit a remarkable situation ... a political crisis without precedence ... galloping insecurity, a morose and precarious socio-economic situation."

After the ceremony, Mr Henry's new Cabinet was formally presented, with the ministers of justice, economy, finance, agriculture and others keeping their positions.

Before Mr Henry officially became prime minister, a crowd of international dignitaries and Haitian officials sat under the shade of pink and white bougainvillea in the yard of the Nation Pantheon Museum in downtown Port-au-Prince to commemorate Mr Moise.

A large portrait of the assassinated president hung behind Mr Joseph as he spoke to mourners.

"The president is dead because of his political and social convictions," he said.

Mr Joseph accused oligarchs and criminals of assassinating the president's character and then the president himself. He called on everyone to seize the moment as an opportunity to find a common purpose and durable, viable solutions.

Mr Moise was shot multiple times, and while officials have arrested at least 26 people in the case, it remains unclear who ultimately was behind the attack. Among the arrested are 18 former Colombian soldiers, five Haitians and three Haitian-Americans.

Police Chief Leon Charles announced four more formal arrests on Tuesday - at least three of them police officers, whose ranks he did not release.

"There was infiltration in the police," he said. "It happened with money."

Mr Charles said authorities were investigating who financed the operation, noting the FBI and Interpol were helping track down US citizens living in the United States who he believes are responsible.