Cities, including the western-central former opposition stronghold of Homs, which played a central role in the uprising and had been under siege by the Syrian army, reported no violence as of mid-morning, activists on Twitter and opposition groups reported.
A pessimistic activist in Homs told al-Jazeera he expected shelling would resume within hours.
Also quiet as of mid-morning was Hama, also formerly besieged by the Syrian military, and Idlib, previously targeted for hosting large numbers of armed groups, including the opposition Free Syrian Army and other opponents of President Bashar Assad's regime, opposition groups said.
Syrian U.N. Ambassador Bashar Jaafari told the Public Broadcasting Service in New York the regime was "on board" with the peace plan but would respond to "counterattacks."
The Free Syrian Army said it would respond to any attack by forces loyal to the regime.
U.N.-Arab League special envoy Kofi Annan, who brokered the peace plan, received written assurances from the Syrian Foreign Ministry regime troops would "cease all military fighting throughout Syrian territory as of 06:00 [local time, 11 p.m. Wednesday EDT] ... while reserving the right to respond proportionately to any attacks carried out by armed terrorist groups," Annan spokesman Ahmad Fawzi said.
U.S. Ambassador to the United Nations Susan Rice described the caveats in the ministry's letter as "worrying" and said the onus was on the Assad regime to halt the violence.
British Foreign Secretary William Hague said that if the cease-fire falls apart, "we will want to return to [U.N.] Security Council in a new attempt to obtain a resolution on Syria, we'll intensify our support for the opposition and we will seek stronger sanctions."
Annan was to brief the Security Council on the cease-fire Thursday.
If the truce holds, the focus was expected to fall on the withdrawal of regime troops, tanks and heavy weaponry, a step that was to have been completed Tuesday, the BBC reported.