The newly appointed president of Syria, Ahmed al-Shara, pledged in a speech on Thursday to create an inclusive transitional government that reflects Syria’s diversity and that will lead the country until it can hold “free and fair elections.”

The speech was Mr. al-Shara’s first public address since his rebel coalition toppled the country’s longtime dictator, Bashar al-Assad, last month and came amid growing unease among some Syrians over his coalition’s plans for leading the country.

“Building the nation is our collective responsibility,” Mr. al-Shara said in the five-minute video address broadcast by Syrian state media. “This is a call to all Syrians to participate in building a new homeland, one that will be governed by justice and consultation.”

The speech came a day after his rebel coalition declared Mr. al-Shara president of Syria for a transitional period and announced a series of other critically important decisions, including nullifying the Constitution and dissolving the legislature and army that were formed under the ousted dictatorship.

The declarations placed control over the formation of a new state firmly in Mr. al-Shara’s hands for the foreseeable future. Mr. Shara has not said how long this transitional period would last, and it was not immediately clear whether his appointment had broad support across a diverse range of armed factions outside of his coalition.

“It’s a very centralized, authoritarian, top-down approach,” said Dr. Salam Said, a Syrian researcher and former lecturer at the Free University of Berlin, describing Mr. al-Shara’s appointment as president. “Progressive Syrians and others are really worried about their role and their participation in this new state,” she added. “They feel left behind in all of these decisions.”

Many Syrians have also questioned whether Mr. al-Shara will be able to reconcile the militant Islamist roots of his rebel group, which evolved years ago from an affiliate of Al Qaeda, with a largely secular state.

In his public address on Thursday, Mr. Shara sought to assuage some of those concerns, emphasizing that the process for establishing the transitional government will involve consultation with people from across Syrian society.

“We must all remember that this is a transitional phase, part of a political process that requires the genuine participation of all Syrians,” he said.

Mr. al-Shara added that as president, he planned to prioritize unifying all Syrian territories, building state institutions and reviving the country’s battered economy. In the coming days, his government will announce a committee to determine the members of a legislative council for the transitional period, he said.

In recent weeks, Mr. al-Shara had come under growing criticism for failing to make a single speech addressing the Syrian public in the nearly two months since his coalition took power in early December. During that time, however, the new leadership publicized a flurry of meetings between Mr. al-Shara and visiting foreign diplomats. On Thursday, Qatar’s emir, Sheikh Tamim bin Hamad Al-Thani, visited Damascus and met with Mr. al-Shara in the first visit by a head of state to the Syrian capital since the rebels seized power.

The way in which the leadership announced crucial decisions on Wednesday — during a private meeting with several other rebel groups in the presidential palace — also drew criticism. Notably absent from the meeting were some Druse militias that effectively control much of southwestern Syria and the Kurdish militia that controls the northeast.

The coalition did not publish any information about which factions were present at the meeting or the process through which they appointed Mr. al-Shara, leaving uncertainty over whether there was a unified front behind these steps.

In the weeks after his rebel coalition swept Damascus, Mr. al-Shara and his allies have spoken of a grand vision for Syria’s new state. Leaders from across society — including activists, intellectuals and opposition figures — would be invited to help build a caretaker government. An inclusive committee would draw up a new Constitution. Elections would be held. Free speech would rein.

That rhetoric was welcomed by many in Syria, a country ruled with an iron fist by the Assad family for more than 50 years. But over time, the euphoria has begun to fizzle.

“They are now deriving their legitimacy from militarily liberating the country,” said Alise Mofrej, a member of the Syrian Negotiation Commission, an umbrella organization for Syrian opposition groups. But Mr. al-Shara needs to assure the Syrian people that he will not establish “a new tyranny,” she added.

Mr. al-Shara’s speech promising inclusivity and broader consultations with Syrian civil society appeared to be an effort to address that criticism. Mr. al-Shara also said that he had conducted intensive legal consultations to ensure his appointment as president had been done “in accordance with legal norms, granting it the necessary legitimacy.”

A spokesman for the interim government’s military operations, Col. Hassan Abdul Ghani, announced on Wednesday that all Syrian militias would be dissolved and integrated into the state. But it was unclear how rebel factions that were not present and have so far refused to give up their arms would respond to that mandate.

Some militias had previously refused to disband and integrate their fighters into a new national army until they had more clarity on the form the caretaker government would take.

Unifying those militias under a single state is among the most pressing challenges Mr. al-Shara’s government faces, experts say, and one made more difficult having likely lost some of their confidence after announcing major decisions on Wednesday without their input.

“While it was expected for him to be announced as transitional president, the way it was done — and the way that some political actors were excluded — cost him some political capital,” said Ibrahim al-Assil, a Syrian adjunct professor of political science at George Washington University who returned to Damascus after the fall of the Assad government.

“I don’t think that was a game changer,” he added. “But it was a step that has people worried about what will come next.”

Reham Mourshed contributed reporting.