Syrian authorities today for the first time acknowledged large-scale violence in the rebellious city of Hamah, where Moslem Brotherhood insurgents have been battling government troops for nearly three weeks.

The admission seemed designed to ease the Syrian public into official recognition that the Hamah uprising has caused heavy casualties and extensive damage to the city.

Despite Syria's use of field artillery and T62 tanks against rebel strongholds, the government-run press and broadcasting stations previously had held to the official version that security forces were merely conducting search operations for arms caches.

The shift came in the form of a cable of support to President Hafez Assad from the Hamah section of the ruling Arab Baath Socialist Party accusing brotherhood fanatics of "annihilating entire families" and "brutally killing all Hamah citizens who refused to open their doors as hideouts for them."

The message, broadcast by the official Syrian radio, clearly pinned the blame on the fundamentalist rebels. It said nothing of casualties caused by Army shelling, estimated by diplomatic sources in Damascus to exceed 1,000 and perhaps even 2,000, compared to several hundred dead and wounded among Army troops trying to put down the uprising.

The fact that it was broadcast at all, however, marked a sharp departure from previous government insistence that foreign press and radio reports--widely listened to by Syrians--had exaggerated the Hamah revolt. But because of these broadcasts and word-of-mouth, many Syrians were well aware of how serious it was despite the attempt to portray it as a routine police action.

Syrian sources reported last week, for example, that the homes of the Hamah police chief and several other local officials were attacked by Moslem Brotherhood rebels in the first days of fighting. These accounts, which alleged that entire familes were massacred, seemed to parallel the accusations in today's Baath cable.

The message also lent credence to reports that have persisted, despite government denials, indicating that fighting is still going on in some sections of Hamah. It said the local Baath militia is cooperating in "the battle of confrontation alongside . . . our comrades in the security forces against the traitorous members of the Moslem Brotherhood gang."

Diplomatic informants in Damascus say several brigades of Syrian troops with heavy artillery and armor have been trying since Feb. 2 to root out Moslem extremists in Hamah's old Hadhir quarter.

It is impossible for outsiders to get firsthand information about the situation in Hamah because roadblocks prevent travelers from entering or leaving the city. Twenty days after the fighting began, the city remains sealed off.

Measured by its duration or its bloodiness, the Hamah revolt is by far the most serious uprising against Assad's 11-year-old rule in almost three years of sporadic sedition blamed on the Moslem Brotherhood and helpers abroad. Because of the seriousness of the uprising, the government has been particularly eager to dampen reports on the Army's intervention to put it down.

Brotherhood statements issued abroad have claimed the Hamah uprising spread to Aleppo and Latakia as part of widespread popular discontent with Assad's government. Information available from Syrian officials and independent diplomatic analysts in Damascus, however, indicates the large-scale violence has been confined to Hamah.

Thursday, however, the rebels scored a blow against the government by sending a suicide attacker into the Information Ministry compound in a panel truck loaded with explosives that killed its driver and caused extensive damage and injuries.

The Moslem Brotherhood attacks Assad's government as repressive, charging it is built around members of the president's Alawite sect. The Alawites, an offshoot of Shiite Islam, make up a 12 percent minority among Syria's 10.5 million inhabitants but are heavily represented under Assad in the Baath Party and the armed forces.