Syria's government on Friday accused the US ambassador to Damascus, Robert Ford, of meeting "saboteurs" in the flashpoint city of Hama and inciting protests against President Bashar al-Assad.
"The US ambassador met with saboteurs in Hama... who erected checkpoints, cut traffic and prevented citizens from going to work," the interior ministry said in a statement.
"The ambassador incited these saboteurs to violence, to demonstrate, and to refuse dialogue," with the government, it added.
Ford visited the city on Thursday, prompting Damascus to hit out at the United States for meddling in Syrian affairs.
"The presence of the US ambassador in Hama without previous permission is 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," the foreign ministry said in a statement.
"Syria warns against such irresponsible behaviour and stresses its determination to continue to take all measures that will bring back calm and stability to the country," it added.
Despite that warning, more than 150,000 people marched in the Syrian hotbed city of Hama on Friday calling for the end to Assad's regime, a rights activist said.
The demonstrators reiterated their "refusal to dialogue with the regime and called for its fall," said Rami Abdel Rahman, chief of the Syrian Observatory for Human Rights.
Hama has been a symbol of opposition since the 1982 crackdown on a revolt by the banned Muslim Brotherhood against then-president Hafez al-Assad, father of the present leader, in which some 20,000 people were killed.
The authorities had told demonstrators to avoid any confrontations and clear the streets so residents could go to work and to avoid what it called a "last resort" military operation, according to Al-Watan newspaper.
The Syrian Observatory said that about 100 families -- or 1,000 people in total -- had fled Hama, where it said Syrian troops had killed 25 civilians since Tuesday, fearing a military crackdown during Friday protests.