CAIRO — For the past two months, President Kais Saied of Tunisia has ridden widespread popular support to ever-higher peaks of power, culminating in a recent announcement that he would essentially rule the country by decree. But he has now begun to face growing opposition, heightening uncertainty over Tunisia’s most serious political crisis in a decade as its economy careens toward ruin.

The rebukes have come from staunch opponents and former allies alike, from political parties and from the media, and even from some of the same supporters who cheered in the streets when Mr. Saied froze Parliament, fired the prime minister and seized power on July 25. On Sunday, at least 2,000 protesters in the capital, Tunis, called for Mr. Saied to end what they called his “coup,” the first major demonstration against his actions in two months.

A joint statement from four political parties, including one that was previously close to the president, said that Mr. Saied was moving toward dictatorship and called on him to end his “exceptional measures,” which he had promised were temporary.

“We consider the president has lost his legitimacy by violating the Constitution,” the country’s powerful general labor union, U.G.T.T., said in a statement on Friday, warning Mr. Saied against concentrating too much power in his hands without dialogue.

Mr. Saied has thrown the North African country’s democracy, the only one to emerge from the Arab Spring protests that began in Tunisia and swept through the region a decade ago, into ever-deepening doubt.

He said in July that his actions were provisional responses to Tunisia’s economic and health emergencies. But the president has only tightened his grip on power since then, ignoring international and domestic pressure to restore Parliament.

On Wednesday, Mr. Saied’s office announced that he would set up a system under which he would essentially rule the country by decree, bypassing the Constitution. It said that he would assume the power to issue “legislative texts” by decree and select the cabinet, even though the Constitution makes Parliament responsible for lawmaking and empowers the prime minister to appoint a cabinet.

As for the Constitution, which Tunisians adopted in 2014 after years of painstaking consultations and negotiations, the announcement said simply that any constitutional provisions that ran counter to Mr. Saied’s new powers were no longer in force. That left in place only the document’s preamble and first two chapters, which deal with Tunisia’s guiding principles and rights and freedoms.

Mr. Saied’s office said that he would take charge of drafting political overhauls and constitutional amendments with the help of a committee that he would appoint.

That item in particular drew alarm from U.G.T.T., the labor union, which was part of a quartet of groups that was awarded the 2015 Nobel Peace Prize for captaining a national dialogue that helped Tunisia’s fledgling democracy survive a political crisis in 2013.

“The amendment of the Constitution and the electoral law is a matter that concerns all components of society,” the union’s statement read on Friday. It called on Mr. Saied to engage in talks rather than monopolizing the power to change the Constitution.

“There is no solution to the current crisis other than consultation, partnership and dialogue on the basis of national principles, Tunisia’s sovereignty, and service,” the union added.

The announcement on Wednesday by the president’s office also said that lawmakers would lose their salaries and benefits in addition to their immunity to prosecution, which Mr. Saied had already lifted. The Tunisian authorities have arrested five members of Parliament over the past two months, including critics of the president, though one, Yassine Ayari, was released last week.

Other targets have included businesspeople and judges, some of whom have been subject to house arrest, travel bans and asset freezes.

At first, many Tunisians were overjoyed to hear of Mr. Saied’s exceptional measures. Pinning their hopes for saving Tunisia’s sinking economy, overhauling the country’s chaotic politics and tackling widespread corruption on a president they saw as incorruptible, they dismissed warnings from Mr. Saied’s political opponents and critics that his actions smacked of dictatorship.

But Mr. Saied has failed to lay out a long-awaited road map for turning the country around and has raised alarm by declining to engage in dialogue with civilian groups or other politicians to determine a way forward.

After two months without results, discontent — or at least impatience — with Mr. Saied’s actions has begun to fester. A small gathering of protesters turned out to demonstrate against him earlier this month; thousands more rallied on Sunday in Tunis.

“Emperor Kais, first of his line,” Sarra Grira, a Tunisian journalist, wrote on Facebook soon after the announcement that Mr. Saied would be assuming greater powers.

But the real test of Mr. Saied’s popularity will be whether he can address the economic misery that threw Tunisia into turmoil in the first place. Struggling with high unemployment, declining living standards and widespread poverty that drives thousands of Tunisians to risk migrating across the Mediterranean to Europe every year, the country has no clear prospects for improvement.

Mr. Saied has paused negotiations with the International Monetary Fund over a bailout without explaining his economic plans, though he has gained popularity among some Tunisians with proposals to force wealthy businesspeople who have been accused of corruption to fund development projects in poorer regions.

“The wall that Kais is hurtling toward and might splatter against is the economy,” said Monica Marks, a professor of Middle East politics at New York University Abu Dhabi who studies Tunisia. “Expectations are so high, and he has everything to do with that,” she added.

“Inevitably, there’s going to be a huge chasm between populist expectations, that are higher than ever now, and the reality of what Kais can actually deliver.”

Massinissa Benlakehal contributed reporting from Tunis.

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