The London-based Syrian Observatory for Human Rights said in a statement heavy gunfire was heard in some neighborhoods of the restive city and that all landline telephones have been cut off, CNN reported.
The opposition Local Coordination Committees of Syria reported tanks had arrived at the city center Wednesday.
Residents told The New York Times the situation in Homs was tense Wednesday, with checkpoints set up on major highways, intersections and at the entrances to the city.
Soldiers and plainclothes armed forces loyal to President Bashar Assad were deployed throughout the city, witnesses said. In addition, snipers were perched on the rooftops of government buildings and schools, firing at pedestrians and cars, witnesses said.
"People are not leaving their houses today as there is gunfire and shooting through out the city," Homs resident Um Nader told the Times. "People are being shot, and bodies were dumped around streets."
Syrian officials Tuesday canceled a visit by the head of the Arab League scheduled for Wednesday, citing "circumstances beyond our control." The Syrian Arab News Agency said the meeting, which was to convey concern about the killing of protesters, would be rescheduled.
The 22-member league had no immediate comment on the last-minute cancellation of Wednesday's meeting.
The cancellation ahead of a possible rescheduling came as U.N. Secretary-General Ban Ki-moon said it was too late for Assad to make amends for his regime's crackdown and U.S. Ambassador to Syria Robert Ford said the international community doubted the Assad regime had the capacity to enact real reforms.
Protesters have demanded an end to Assad's regime for months despite violent crackdowns by security forces. Assad has said he is open to reforms and argued security forces are protecting the citizenry from thugs and terrorists.
Ford said he accepts that Syrian security forces have died during the protests, but in far fewer numbers than protesters. Because of its "preponderance of arms and force," the Syrian government "bears the responsibility for the violence," he said on the U.S. State Department's Facebook page for Damascus.
"We support the right of Syrians to protest peacefully," Ford said. "Peaceful protesters are not 'terrorists.'
"Given the extent of the government's brutality, neither the Syrian protest movement nor the international community will believe that this Syrian leadership desires or is capable of the deep, genuine and credible reforms that the Syrian people demand."
On Tuesday, at least 10 people were killed nationwide, the Syrian Observatory for Human Rights said.
Residents in Aleppo joined the anti-government protests Tuesday after a local well-known scholar died, CNN reported. Officially, Ibrahim Salqini died of a heart attack, but activists have questioned the circumstances of his death.
Mourners attending Salqini's funeral chanted "better death than humiliation," activists said, and Syrian security forces opened fire when the protesters reached the cemetery. An activist told CNN several people were injured and many others detained.
International pressure on the Assad regime also has grown. The European Union has banned oil imports from Syria and the United States invoked another round of economic sanctions as well. The Arab League last week called for restraint and an end to the violence.
More than 2,200 people have died in Syria since February, the United Nations said.