"The tally that we have received from various activists in Homs since the shelling started at six this morning is 50, mostly civilians," the group's Catherine al-Talli told Reuters.
The assault follows the veto by Russia and China on Saturday of a United Nations Security Council resolution that called for a change of Syrian leadership. Critics of the double veto contend that it would embolden Syrian president Bashar al-Assad to step up attacks against the opposition.
Human rights groups say more than 7,000 people have died since the start of a popular uprising in March. The UN's last tally in December was of 5,000 dead.
Talking to reporters on Sunday during a visit to Bulgarian capital Sofia, US secretary of state Hillary Clinton said she would work will allies outside of the UN to pressure Assad to resign.
In the highest casualty count since the the beginning of the conflict, reports said that more than 200 people died during a Friday artillery assault on Homs.