Thomas Thabane's All Basotho Convention (ABC) party won snap elections on June 3 but failed to get an outright majority.
Thomas Thabane's All Basotho Convention (ABC) party won snap elections on June 3 but failed to get an outright majority, leading it to negotiate joint rule with the Alliance of Democrats (AD), Basotho National Party (BNP) and Reformed Congress of Lesotho (RCL).
But his inauguration was marred by the murder of his estranged wife Lipolelo, 58, who was gunned down on Wednesday night in a village near the capital Maseru.
Thabane's new coalition government comes into office with the hope of ending the political uncertainty of party splits and fragile coalitions that have long dogged the mountain kingdom.
He said it was disturbing that Lesotho was increasingly becoming notorious for its failed governments and that it was time to change.
"I sincerely hope that political reforms will bring stability in Lesotho and ensure that Lesotho governments last for a normal five-year term," he said in his inaugural speech.
The new alliance replaces the government of Pakalitha Mosisili, a seven-party coalition plagued by infighting and corruption.
Mosisili's government was toppled in March and elections triggered after opposition parties called a vote of no-confidence which he lost.
The Thabane-led alliance won 63 of the 120 seats in parliament, while outgoing Mosisili's Democratic Congress (DC) scored just 30.
At the ceremony, which was held inside a stadium in Maseru, Thabane thanked his predecessor "for facilitating the smooth transfer of power" and said the two would "remain friends forever."
Thabane, 78, previously served as premier of this nation of two million people after the 2012 elections but was forced to flee to South Africa following an attempted military coup two years later.
In his speech, Thabane commented for the first time on the killing of his estranged wife.
"I am mourning her death, and the senseless killing of people like this... is one of many challenges that I am faced with as a new prime minister," he said.
The gunning down of Thabane's wife just two days before he took office created a sense of confusion in the tiny poverty-stricken country which is completely surrounded by South Africa.
Thabane secured victory just four months after his return from South Africa where he fled in August 2014 claiming there was an army plot to kill him.
He only returned after the removal of army chief Tlali Kamoli who led the putsch.
Thabane's supporters had earlier booed the outgoing premier Mosisili and the country's security chiefs as they separately entered the 40,000-seater stadium which was filled to capacity.
Known as Africa's Switzerland because of its mountainous scenery, Lesotho has a long history of political instability having suffered coups in 1986 and 1991.
"Let's all support this government to create an enabling environment for Lesotho to become a lawful country," said Thabane.
"We can also achieve this through a national unity and reconciliation dialogue."
The investiture was attended by Zambia's President Edgar Lungu, South Africa’s Vice President Cyril Ramaphosa and Swaziland Prime Minister Barnabas Dlamini.
Lungu described democracy as an expensive exercise.
"Lesotho needs to decide how best (it) can make democracy a bit cheaper and channel (its) resources into other developmental issues," he said.