BANGKOK — In its first test at the polls, Thailand’s military government was winning overwhelming approval Sunday evening of a new constitution that aims to reduce the power of political parties and extend the influence of the military.

With 94 percent of the ballots counted, voters were approving the military’s proposed constitution by a wide margin, according to preliminary returns issued by the election commission. A companion ballot measure that would give the military junta the authority to fill the Senate with its appointees was also easily winning voter approval.

“The voting in all areas was smooth,” said Supachai Somcharoen, chairman of the Thailand Election Commission, after the polls closed. “No disturbances occurred at all.”

The constitution would be the country’s 20th in 84 years.

“The politics from now will be more compromising, more negotiating,” Yuthaporn Issarachai, dean of political science at Sukhothai Thammathirat Open University, said in a television interview. “It won’t be politics ruled by the majority. So we will expect to see some adjustments from the political parties. We may see the switching of sides and some negotiations.”

The junta seized power in 2014 and brought temporary peace to the country after years of clashes between political factions. The military leaders have pledged to hold parliamentary elections next year and return power to civilian leaders no matter what the outcome of the voting on Sunday.

Human rights groups challenged the legitimacy of the referendum because of restrictions on campaigning that prevented opponents of the proposed constitution from getting out their message.

The junta limited public assemblies and threatened long prison terms for people who spread information that it deemed false. More than 120 people were arrested for violating campaign rules in the weeks leading up to the referendum, according to Human Rights Watch.

Thailand has long been divided between the rural poor, mainly in the northern and northeastern parts of the country, and the urban middle class, leading to years of protests and of clashes between the two factions.

Two populist prime ministers, Thaksin Shinawatra and his sister, Yingluck Shinawatra, were elected with the support of rural, northern voters. But Mr. Thaksin was ousted by a military coup in 2006, and Ms. Yingluck was ousted by a court ruling just before the 2014 coup.

Opponents have long accused both siblings of being corrupt. Mr. Thaksin, who lives in self-imposed exile, was convicted in absentia in 2008 of violating conflict-of-interest rules in a land deal. Ms. Yingluck is on trial for criminal negligence in managing government rice subsidies for poor farmers.

Adding to Thailand’s precarious political situation has been the lingering illness of King Bhumibol Adulyadej, 88, who has been hospitalized for more than a year. Analysts say the military sees its role as ensuring stability as the country prepares for the expected succession of Crown Prince Vajiralongkorn.

Thailand has experienced a repeated cycle of elections, coups and new constitutions since the absolute monarchy was abolished in 1932.

The proposed constitution was designed to shift the balance of power away from major political parties and give a greater voice to medium-size parties under a new formula for awarding seats in Parliament. Experts say the system was designed so no single party would have control and so the country would be ruled by a coalition of political parties.

Approval of the second measure on the ballot would give the junta the power to appoint the 250 members of the Senate to five-year terms and would give the senators a role in selecting the prime minister, which previously was left to the House of Representatives.

The biggest loser under the new system would probably be Ms. Yingluck’s Pheu Thai party.

“For the political situation in the future, it will be a coalition government,” a former election commissioner, Sodsri Satayathum, told reporters. “In the upcoming election, there won’t be a single party that wins the majority.”

She questioned how effective such a coalition government would be.

“The government will find it hard to run the country,” she said.

In Bangkok, voters who cast their ballots for the junta’s proposals said the two measures would help maintain stability and reduce corruption in government.

Sunee Nateethong, 50, a businesswoman in the capital, said she voted for both because she favored a continuing role by the military to reduce corruption and maintain stability.

“It is better than politicians running the country,” she said. “It’s good to have the military babysitting the government for the next five years.”

Sikarin Kanoksikarin, 44, who works for a United States food producer, said it was better for the country to have the military restore democracy gradually, as the new constitution would.

“To change the political system takes time,” he said.

But voters who cast their ballots against the junta’s proposals said they were dissatisfied with its management of the economy and were tired of the military taking power.

“I don’t like dictatorship,” said Putiporn Sa-ngangam, 21, an accountant. “This is our country’s cycle. We have an election and a coup over and over again. I think the previous constitution was good.”

Paparat Yurod, 46, who comes from northeastern Thailand but works in a Bangkok laundry, said voters should be able to decide for themselves who runs the country, without military interference.

“We are hoping to get democracy soon,” she said. “Even if we have a bad government, we will have another election and can decide if they continue in power.”