TUNIS — The authorities in Tunisia have arrested nearly 800 people who were protesting tax increases and other government policies after a series of clashes with the police that have alarmed observers of Tunisia, the only country to emerge from the turmoil of the Arab Spring with the semblance of a stable democracy.

Although some of those who were arrested were released, the number of detentions drew criticism from human rights groups.

“We’re concerned about the high number of arrests,” Rupert Colville, a spokesman for the United Nations high commissioner for human rights, said Friday in Geneva. “Some 778 people we understand have now been arrested since Monday, and around a third of those arrested were between the ages of 15 and 20.”

He added: “We call on the authorities to ensure that people are not arrested in an arbitrary manner and that all those detained are treated with full respect for their due process rights and are either charged or promptly released.”

Three local leaders of the Popular Front, the main leftist opposition party, were detained in the town of Gafsa, reportedly for setting fire to a government building. The party said its leaders had been targeted in a political campaign that was “reproducing the methods of the oppressive Ben Ali regime,” a reference to Zine el-Abidine Ben Ali, the longtime dictator who was toppled in 2011.

The protests that began on Monday have been largely aimed at a series of austerity measures and tax increases that went into effect on Jan. 1 and have quickly rippled through the economy, which is plagued by high inflation and high unemployment.

Although the protests appeared to be ebbing on Friday, there were calls for new political agitation on Sunday — the seventh anniversary of Mr. Ben Ali’s overthrow.

The police remained out in force in Tunis. There were several scuffles. Mayssa Oueslati, a woman from Tunis, shoved a police officer, telling him to leave her alone. She said he had told her, “We know you from before — be careful.”

She said the police were practicing familiar tactics of intimidation: “We know their ways, how they watch us when we go in a cafe or just protest. There is a gap between policemen and the youth. You can see it in how they treat soccer fans in stadiums, not just with protesters.”

Several of the protesters have rallied around the slogan We Do Not Forgive, an anticorruption campaign that began in 2015.

Amna Guellali, a Tunisia researcher at Human Rights Watch, said the number of police officers was very high compared to the number of protesters, and expressed concern about whether the protesters had adequate freedom of movement.

In a news release, Reporters Without Borders said that journalists covering the protests had been intimidated, including a French reporter who was taken from his home and escorted to a police precinct for questioning. The police asked the reporter for the contacts of journalists in Tebourba, a town with angry protests this week, and demanded to know whom he had spoken with.

One death has been attributed to the protests, that of Khomsi el-Yerfeni, who died Monday in Tebourba. Onlookers said that he might have been struck by a police car, but the authorities said that he was not hit and that he had suffered from chronic shortness of breath. An autopsy has been conducted but not yet released.

With its relatively stable democracy, Tunisia has been a rare success story following the Arab Spring uprisings that began in 2010 in Tunisia and soon spread to Egypt, Libya, Syria, Yemen, Bahrain and other nations.

The International Crisis Group, an organization committed to preventing and resolving deadly conflicts, warned in a report on Thursday that the achievements of Tunisia’s revolution risked being overshadowed by political tensions and a nostalgia for strong, centralized government.

The group added: “Tunisia has a special responsibility to stand up to this tendency, to avoid new jihadist violence, to prevent a return to political polarisation and to sustain its role as the sole Arab state sticking to a peaceful, more democratic course since the 2011 Arab Uprisings.”

The group urged the strengthening of Tunisia’s political institutions, the creation of a constitutional court and the establishment of independent oversight bodies and called for long-delayed local elections to be held this year.

But unrest in Tunisia has both political and economic dimensions. While a group of Tunisian organizations was awarded the Nobel Peace Prize for helping to manage the country’s transition to democracy, reviving the economy has been far more difficult.

There are also big regional differences. In towns like Thala, the dominant concerns are the economic despair of young people grappling with high inflation and a lack of jobs; in the more affluent capital, Tunis, young protesters have been focusing on reforms in governing.