Protests have built up into nightly affairs in Syria’s sprawling capital and activists are pressing boycotts against Syrian insiders, as action against the government moves closer to the political and administrative core of President Bashar al-Assad’s regime.

Protests have started to build in new areas of Damascus and gain breadth across neighborhoods in the city over the past few weeks. Less visibly, young activists started publicizing lists of brands and companies distributed or owned by people they say are close to the ruling regime to boycott.

The boycott list includes brands of cigarettes, canned tuna and dairy products as well as taxi companies and cafes. The list includes several companies linked to Rami Makhlouf, a cousin of the president who has been the target of U.S. and European Union sanctions and who last month vowed to retire from business.

“The solution to mobilizing Damascus is to economically strangle the bourgeoisie and business class that benefit from the regime,” said one activist, who was disappointed to learn his favorite milk brand was controlled by Mr. Makhlouf.

Damascus and Syria’s second city, Aleppo, are keys to the survival of Mr. Assad’s regime. Together, they are home to over half of Syria’s population of at least 21 million. Analysts say that should mass protests mobilize there, the two cities hold the potential to tip Syria into what the International Crisis Group has called a “slow-motion revolution.”

But in Damascus, where many remain loyal to Mr. Assad and others are reluctant to join protests that risk destabilizing their country, antigovernment activists have adopted lower-key tactics, like the boycotts, to draw in supporters.

Protests in the capital haven’t exploded like in other large cities Homs or Hama, where tens of thousands have demonstrated around public squares. Tanks surrounded Hama on Sunday, ending a month-long spree of protests free of regime oversight. Security forces continued to press their campaign there, with 24 people killed on Tuesday and Wednesday and over 700 detained on Wednesday and Thursday, according to global campaigning organization Avaaz.

Damascus has seen small protests since the early weeks of the four-month-long uprising, both in the belt of underdeveloped suburbs around the city and in urban neighborhoods aside from the capital’s wealthiest districts. The military has locked down at least nine suburbs at various points and parts of Douma, Daraya and Moadamiyeh remain under a security siege, residents and activists say.

In the past few weeks, protests have become larger, closer to central Damascus and as frequent as nightly. This past week, two separate protests marched through central Baghdad Street, not far from the parliament building.

Violence against protesters is partly what has motivated more people to join protests, as in other spots across Syria, city residents and activists said. In a show of the regime’s willingness to crush dissent even at a centrally located college campus, security forces stormed Damascus University on June 21, killing one student.

Students described thugs breaking down dorm doors and dragging women out of their beds, just a day after Mr. Assad had delivered a speech at the university acknowledging protesters had legitimate demands and promising reforms.

With pervasive security and intelligence surveillance making it difficult to organize, activists have turned to less-overt expressions of dissent. Unlike calls for a nationwide general strike—which have fallen flat in Damascus and Aleppo—they hope boycotting products and places will allow more people to support the protesters.

Some businessmen are already donating money to protesters or families of the injured outside the capital. Activists say some of their laptops have been provided by Damascene businessmen, who they say maintain a public proregime line to protect their business interests.

“Many, but by no means all, Damascenes have the most to lose economically from the collapse of the regime, compared to those outside the capital, so they are likely to be the last to protest en masse,” a senior Western diplomat in the capital said.

Syria’s economy is already sputtering, as a near-total dropoff in tourism leaves hotels deserted and shopper reluctance has stores closing for what has traditionally been busy evening hours. If a targeted boycott of businesses aggravates those woes and protests continue to grow, analysts say members of Damascus’s business community could quickly switch sides to cut their losses.

Protests are expected to grow in August, when people gather at mosques for prayer daily—rather than weekly—during the Muslim holy month of Ramadan.

For now, protesters have abandoned attempts to gather at the capital’s two large squares, Omayyad Square and Abassin Square, after a march toward Abassin in April was violently dispersed by security forces.

Omar Idlibi, a spokesperson for the activists’ Local Coordinating Committees, said they don’t aim to settle around a public square in the capital, as Egypt’s protesters did. They have avoided overnight sit-ins, hoping instead to wear out the regime’s military and security with the spread of protests across the country.

“Damascus is likely to be the last place where there will be large scale antiregime protests,” the diplomat said. “The regime’s continued strong grip on [Damascus and Aleppo] also sends a powerful symbolic message to Syrians and outsiders that the regime is still in control of the country.”

The capital’s long-held loyalty to Mr. Assad is pronounced in a growing number of portraits and posters of the president around the city. Pro-regime rallies have also grown in recent weeks.

Some say antiregime protesters are still limited to disgruntled residents of cramped, lower-class neighborhoods even when they march through the boutique-lined streets of al-Shaalan.

Others say surprising constituencies have joined. Unable to gather in public squares, secular activists and even Christians have found sanctity in mosques as a place to gather for protests.

“My Christian and Communist friends and classmates come to the mosque with me every week, just to protest after,” a university student said. “They don’t know how to pray, but they ask me what to do when we’re on the way.”

—A reporter in Damascus contributed to this article.Write to Nour Malas at nour.malas@dowjones.com