The continuing crackdown on opponents of Bashar al-Assad came as the president's regime accused US Ambassador Robert Ford, who had visited the city, of inciting violence.
The casualties included two protesters killed in a central commercial neighbourhood of Damascus, Abdel Karim Rihawi, who heads the Syrian League for Human Rights, told AFP.
"Security forces shot dead two demonstrators in the neighbourhood of Medan in Damascus, and another was killed in the area of Dmeir" east of the capital, Rihawi said.
In the central city of Homs, "at least five people were killed in the Al-Khalidya neighbourhood by security forces who opened fire against demonstrators," he added.
The Syrian Observatory for Human Rights said at least 24 people were injured in Homs and that some people were gravely wounded.
Observatory chief Rami Abdel Rahman said at least 450,000 Syrians rallied after Friday prayers in Hama under the banner "No to dialogue" with Assad's regime.
The Hama demonstrators reiterated their "refusal to dialogue with the regime and called for its fall," he said.
Both US envoy Ford and French Ambassador Eric Chevallier visited Hama on Thursday.
On Friday, the regime accused Ford of meeting "saboteurs" there and inciting anti-Assad protests.
"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 ambassador incited these saboteurs to violence, to demonstrate and to refuse dialogue" with the government, it added.
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."
In Paris, foreign ministry spokesman Bernard Valero said Chevallier went to Hama "to show France's engagement with the victims, the civilian population."
"In particular, he went to one of the main hospitals in the city where he met medical teams, the wounded and their families.
"France repeats its concern over the fate of the inhabitants of the city of Hama, and its condemnation of the violence being carried out by Syrian authorities against the protesters and the civilian population."
Elsewhere, security forces used live rounds against protesters in the coastal city of Banias.
"Protesters left the mosques in the southern districts of the city and dozens were arrested," Abdel Rahman said.
Further north on the coast, in Latakia, more than 1,000 protesters were dispersed as they left Al-Rahman mosque, an activist there told AFP.
"Armed security services came in more than 20 cars, dispersed the protesters and arrested some of them," the activist said.
In Aleppo, a regime stronghold, "thousands of people rallied at Seif al-Dawla and Salheddine. They were attacked by security forces and a pro-regime militia," a witness told AFP.
Protests also took place in Saqba and Jdeidet Artuz, while 7,000 protesters filed through Qatana north of Damascus, activists said.
In the northern town of Raqqa, "security forces fired live rounds and tear gas to disperse protesters, at least one of whom was hurt," said an activist who requested anonymity.
Thousands also marched in Syria's Kurdish regions.
Between 4,000-5,000 took to the streets of Qamlishi, as did thousands more in Amuda, expressing solidarity with Hama and calling for the fall of the regime, rights activist Radif Mustapha said.
Before Friday's protests, the Facebook group Syrian Revolution 2011 had urged "No to dialogue: What dialogue (is possible) when blood has been spilled, while the towns are besieged? The people want the fall of the regime."
It added: "No to dialogue with the assassins, the still-warm blood of the martyrs is calling us. We are legitimacy... dialogue with the regime is useless."
Residents of Hama and Homs staged a general strike ahead of Friday's protests, according to Abdel Rahman, while Syrian Revolution 2011 called for the boycotting of businesses that trade with Assad's regime.
"Don't pay for the bullets that are being used to kill us", the group urged its online followers, also calling on activists to march on the Turkish border on July 16 in support of "the refugees, exiles and victims" of the unrest.
Rights groups say that more than 1,300 civilians have been killed and 10,000 people arrested by security forces since mid-March when the anti-government protests erupted.