President George W. Bush criticized the visit, saying it sends mixed signals to Damascus.
Pelosi's visit was a new high-profile challenge to Bush by the majority Democrats in Congress, who are taking a more assertive role in influencing policy in the Middle East and the Iraq war.
The United States accuses Syria of interfering in Lebanon, allowing Iraqi Sunni insurgents to operate from its territory and sponsoring terrorism for its backing of the Hamas and Hezbollah militant groups, charges Syria denies.
The Bush administration has resisted calls to open direct talks with Damascus to convince it to help ease the crisis in Iraq and bring progress in the peace process with Israel.
Soon after Pelosi's arrival in Damascus, Bush denounced the visit.
"A lot of people have gone to see President Assad ... and yet we haven't seen action. He hasn't responded," he told reporters at a Rose Garden news conference. "Sending delegations doesn't work. It's simply been counterproductive."
He said Assad had not reined in Hamas and Hezbollah and has acted to destabilize the democratically elected government of Lebanon.
Pelosi, a California Democrat, made no comment on Bush's remarks, instead heading from the airport to Damascus' historic Old City for a tour to meet Syrians face-to-face. She is to meet Assad on Wednesday.
Draping a flowered scarf over her hair and donning a black abaya robe, Pelosi visited the 8th Century Omayyad Mosque, shaking hands with Syrian women inside and watching men in a religion class sitting cross-legged on the floor.
She stopped at an elaborate tomb inside the mosque said to contain the head of John the Baptist and made the sign of the cross in front of the tomb. About 10 percent of Syria's 18 million people are Christian.
At the nearby outdoor Bazouriyeh market, Syrians crowded around her, offered her dried figs and nuts and chatted with her. She strolled past shops selling olive oil soaps, spices and herbs, and at one point bought some coconut sweets and eyed jewelry and carpets.
The tour sought to highlight the Democrats' stark differences with the Bush administration's policy of shunning Damascus, which they have depicted as a failure.
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