Opposition activists reported five deaths in the central city of Homs, two in the capital's commercial neighbourhood Medan and six in the area of Dmeir, east of Damascus.
In Homs, "at least five people were killed in the Al-Khalidya neighbourhood by security forces who opened fire against demonstrators," said Abdel Karim Rihawi, president of the Syrian League for Human Rights.
"Security forces shot dead two demonstrators in the neighbourhood of Medan in Damascus." London-based Rami Abdel Rahman, who heads the Syrian Observatory for Human Rights, said six people were killed in the area of Dmeir and that at least 24 people had been injured in Homs, some of them gravely.
Rahman said a record 450,000 Syrians rallied after Friday prayers in Hama under the banner "No to dialogue" with Assad's regime and called for its fall.
Both US envoy Robert Ford and French Ambassador Eric Chevallier visited the city on Thursday.
"The US ambassador met with saboteurs in Hama... who erected checkpoints, cut traffic and prevented citizens from going to work," an interior ministry statement said.
The foreign ministry called Ford's presence in Hama "obvious proof of the implication of the United States in the ongoing events, and of their attempts to increase (tensions), which damage Syria's security and stability."