Two weeks after an electoral upset that ousted Sri Lanka’s long-reigning strongman, the country’s new leaders are racing to investigate allegations of corruption under the previous regime and are rushing to enact legal and constitutional changes they say are aimed at reestablishing rule of law.

“This is going to be a huge, huge challenge,” said Ranil Wickremesinghe, who became prime minister after the defeat of former President Mahinda Rajapaksa. Mr. Wickremesinghe and the new president, Maithripala Sirisena, have promised to complete a far-reaching revamp of the government within 100 days.

In an interview with The Wall Street Journal, Mr. Wickremesinghe said one of his most urgent priorities was to depoliticize the police and “dismantle the apparatus that was built up” under Mr. Rajapaksa to stifle dissent. His government has called on journalists and intellectuals who have fled the country to return.

A 65-year-old veteran politician who has served as premier twice before, Mr. Wickremesinghe also said he is determined to work toward a lasting political settlement with the country’s ethnic Tamil minority. Mr. Rajapaksa’s government crushed a decades-long Tamil separatist insurgency in 2009.

January’s election was “a vote for change—change that includes reconciliation,” he said. The prime minister said the government would push to increasingly empower the provincial government in the north, where Tamils outnumber the island nation’s majority Sinhalese.

In tackling corruption allegations, Mr. Wickremesinghe, who works in a small and sparsely furnished office on the ground floor of Temple Trees, Mr. Rajapaksa’s former residence, said he and other cabinet ministers are still struggling to find out the exact terms of deals struck by the previous government with Chinese lenders and contractors.

Agreements for some mega-projects initiated under Mr. Rajapaksa were never made public and often there was no competitive bidding process, ministers said. Some contracts appear to have been revised after they were signed and allegations of bribery and kickbacks abound, ministers said.

Dinesh Gunawardena, a member of parliament and an ally of Mr. Rajapaksa, said parties in the former governing alliance were open to a “transparent investigation” but said the new government has yet to produce any clear evidence of corruption. “They must have concrete facts and stop mudslinging.”

Ganesh Dharmawardena, director general of Sri Lanka’s Commission to Investigate Allegations of Bribery or Corruption, said 40 to 50 complaints of “serious political corruption” have been received since the elections. “There’s been a huge increase.” So much so, he said, that he needs to hire more investigators.

Mr. Dharmawardena attributed the uptick to a climate where people felt safe enough to make allegations as well as political score-settling.

Among those under investigation is the former governor of the central bank. The ex-governor, Ajith Nivard Cabraal, said Friday those making the allegations wanted “to vilify and discredit” him and would be “unsuccessful.”

In the case of a massive Chinese-funded real-estate development that would create hundreds of acres of landfill in the waters off Colombo, officials have yet to determine if any environmental-impact study had been completed, Mr. Wickremesinghe said.

“We will take whatever necessary action,” he said, if it is determined “the project is not in conformity with the law.”

The prime minister said the government is also scrutinizing a Chinese-funded airport and port built in Mr. Rajapaksa’s home district “both for the high cost as well as for the fact it has not brought benefits to Sri Lanka.” He said the government was looking for ways to make the projects sustainable. Mr. Gunawardena, the backer of Mr. Rajapaksa, said the projects were “good for the country.”

Other infrastructure deals are also being reviewed. “There is a big cry of corruption regarding many of the road projects that have been funded by China,” Mr. Wickremesinghe said. “There was a heavy markup and that money had gone to members of the previous administration.”

Mr. Rajapaksa’s brother, Basil, who served as minister of economic development, has said neither the family nor other politicians had benefitted from infrastructure projects.

Despite the new government’s investigation of past deals, the new premier said: “We’d like to continue that close relationship with China.” He also said the government would work hard to make Sri Lanka more attractive to foreign investors. “We will ensure there is a level playing field,” he said.

On Friday, China’s ambassador to Sri Lanka, Wu Jinaghao, said China would “support the new government to develop their polices” and looked forward to continuing cooperation.

Eran Wickramaratne, the deputy minister for highways and investment promotion, said that “clearly, contracts that have been made will be honored,” but he said that if there is leeway for terms to be adjusted “to get something more reasonable, we will try to discuss it.”

Over the next several months, Messrs. Sirisena and Wickremesinghe have pledged to amend Sri Lanka’s constitution to strip the presidency of many of its executive powers and move toward a parliamentary system with a prime minister as the head of government.

Afterward, they have promised to call for new parliamentary elections, raising the question of whether the broad opposition coalition that came together to topple Mr. Rajapaksa in January will be able to win again and continue governing.

The opposition beat the former president by attacking what they called his slide toward authoritarianism and the concentration of power in the hands of his family. In addition to the brother who served as economic-development minister, another brother was secretary of defense. A third was speaker of Parliament.

During his time in office, Mr. Rajapaksa greatly expanded the Temple Trees compound that served as his residence. One new, multistory building included an enormous wood-paneled presidential office, hung with paintings depicting scenes including a Sinhalese king leading his troops.

The president’s chair was flanked by a pair of traditional maroon-colored parasols known as sesath, symbols of status once borne by the attendants of nobles. Behind it was a sprawling marble bathroom with a flat-screen television fixed to the wall across from the toilet.

Mr. Wickremesinghe said he is confident Sri Lankans are ready for a different kind of politics. During the election campaign, Mr. Rajapaksa’s camp argued that weakening the power of the presidency would “mean chaos.” But, he said: “When people see us implementing our program, that myth will be shattered.” - The Wall Street Journal