Syrian security forces are summarily executing people on the streets of Hama, a human rights group said Thursday, raising fears that bloodshed could escalate dramatically in the besieged city even as world condemnation of the violence continues to mount.

Activists said an initial count suggested 100 people were killed in Hama on Wednesday, bringing to more than 200 the number who have died since the military first moved Sunday to crush the revolt there as part of a broader offensive to quell the nationwide uprising against the rule of President Bashar al-Assad.

An almost-complete communications blackout kept Hama cut off from the outside world for a second consecutive day, making it hard to verify information or obtain an exact casualty toll. Syrian authorities suspended cellphone services, land lines, electricity and water when tanks rumbled into the city center on Wednesday, drawing an international outcry and the first statement from the United Nations condemning the brutal suppression of protesters since the revolt began in March.

But reports filtering out from residents with satellite phones and people who managed to flee painted a grim picture of a city under siege, with tanks deployed at every major intersection, bodies lying uncollected on the streets and people burying the dead in gardens.

The human rights group Avaaz quoted a doctor at a city hospital as saying that at least 109 people were killed in bombardments and shootings during Wednesday’s onslaught.

Wissam Tarif, an activist with the group, said he had spoken by satellite phone to the doctor, who counted the 109 bodies on a tour of hospitals and clinics in three neighborhoods. A number of the victims he had received at his own hospital had been shot at close range in the head, Tarif said, leading the doctor to conclude that at least some of the victims had been executed.

“He doesn’t know if they are random executions or if they are targeting known activists,” Tarif said. “He says the crackdown is being taken to a different level, with the security forces shooting everything.”

He said the hospitals are overwhelmed by scores of injuries and are running out of blood. He added that doctors are being forced to make decisions about whom to save and whom to allow to die based on the complexity of people’s injuries and the time it would take to perform surgery.

The revolt in Hama had seen Syria’s fourth-largest population center transformed into what activists called a “liberated” city. Security forces had retreated, enabling massive anti-government protests to proceed unhindered in the city for weeks.

The hundreds of thousands of people who attended the rallies inspired protesters elsewhere in the country, and U.S. Ambassador Robert Ford visited in early July in a gesture of American support. Hama’s reputation as the site of a major massacre in 1982 in which at least 10,000 people died, during the rule of Assad’s father, had led many residents to hope that the city was simply too sensitive a site to attack.

There was still no indication Thursday that the current crackdown is approaching that scale. A resident contacted by satellite telephone said tanks and troops were stationed at major roads, at intersections and on the rooftops of strategic buildings but still had not ventured deep into residential neighborhoods.

The worry, he said, is that they will — and perhaps go house to house, hunting down opponents, as they did in 1982. He said he ventured out of his home in a central neighborhood and saw the bodies of two women lying unattended on the street, because it is too dangerous for ambulances to reach them.

“The fear that we have now is if they go into small streets to conduct house-to-house searches,” said the man, a dentist who asked that his name not be used because of concern for his safety. “Everyone is afraid of that, because they have lists of wanted names.”

With so many residents in the city of 700,000 having participated in the demonstrations, no one can be sure whose names might be on the lists. Hundreds of people have fled Hama in recent days, but most have been deterred by the risk of being captured at a checkpoint.

A woman who fled Wednesday said she was able to slip out of the city with her husband after driving around for an hour in a taxi searching for a back route that would evade checkpoints.

“Some people are able to leave and some are not. It depends on their luck,” she said in a telephone interview conducted on the condition of anonymity because she feared for her safety. “They have lists on checkpoints with names of those who participated in the demonstrations, and some of those who tried to escape were shot to death.”

In its statement, the U.N. Security Council gave the Syrian government seven days to halt the violence or face further action, perhaps a full-scale resolution. The United Nations has been slow to respond to the crisis in Syria in part because of opposition by Syria’s allies Russia and China.

But on Thursday, Russian President Dmitry Medvedev indicated that Russia was toughening its stance.

If Assad fails to open negotiations with the opposition and adopt reforms, “he will face a sad fate,” Medvedev said in comments quoted by the Russian press. “And in the end we will also have to make some decisions. We are watching how the situation is developing. It’s changing, and our approach is changing as well.”

Assad issued a decree Thursday enshrining a law permitting multiple political parties, but when the law was adopted by the cabinet last month its provisions were widely derided as meaningless because they are so restrictive.

A special correspondent in Beirut contributed to this report.