The crowds, waving Syrian flags and carrying pictures of their president, chanted: "With our souls and our blood we will sacrifice ourselves for Asad."
Demonstrators burned American flags in another part of Damascus and riot police set up barriers near the US embassy.
The Damascus demonstrations, which denounced mounting foreign pressure on Syria and pledged to stand by Lebanon, came a day after hundreds of thousands of Lebanese showed up to a pro-Syrian rally called by Hizb Allah in Beirut, dwarfing previous protests demanding Syria quit Lebanon.
"Yesterday, the true face of Lebanon was shown, unlike those small rallies staged by the opposition," engineer Malik Abd Allah, 56, said.
"I am here to show my support for Syrian policy."
The 14 February killing of former Lebanese prime minister Rafik al-Hariri sparked angry protests in Beirut against Syria, which was blamed for the assassination.
The accusations, denied by the Syrian government, have offended ordinary Syrians.
Al-Asad announced on Saturday that Syrian troops would withdraw from Lebanon, which they entered early in Lebanon's 1975-1990 civil war, but said Damascus would still have a role as its neighbour.
"Syria will not abandon its responsibility towards Lebanon," read one banner at the rally.
"Syria is the shrine of the free and will always be the castle of steadfastness and the fortress of Lebanon," read another.