(CNN) -- Syria's president made a rare public appearance Wednesday.
Bashar al-Assad visited the Umawyeen Electricity Station at Tishreen Park in Damascus to commemorate Workers' Day. He congratulated the station's staff and all other Syrian workers.
The visit came amid more violence in the civil war, a 2-year-old conflict fueled by a grass-roots revolt against the government. At least 50 people were killed Wednesday, the opposition Local Coordination Committees of Syria said.
The opposition Syrian Observatory for Human Rights said regime aircraft bombarded targets in the Damascus countryside and Aleppo province.
The group said an improvised explosive device detonated near a police headquarters in Damascus, injuring a number of people, including children.
A giant plume of thick, gray smoke rose over central Damascus, hanging over the skyline. It was not immediately known if there were any fatalities.
According to the state-run Syrian Arab News Agency, four people, including a 10-year-old child, were killed, and more than two dozen wounded in a series of bomb blasts in Damascus
Two of the explosive devices were planted on bicycles, SANA said, blaming the violence on terrorists.
Hezbollah and Syria
Hezbollah has threatened to intervene further in Syria if there is any threat that the regime could fall to the United States, Israel or extremists, the Lebanese militant group said.
The radical Shiite group, which the United States and Israel call a terrorist organization, has said it is in Syria only to protect Lebanese in the country and to protect holy Shiite shrines from the warfare.
But reports emerged months ago that Hezbollah fighters are battling Syrian rebels near the Lebanese border.
On Tuesday, Hezbollah leader Hassan Nasrallah said the group could step up its presence in Syria and issued an ominous warning to opposition members backed by foreigners who may try to overthrow al-Assad's regime.
"Syria has real friends in the region and the world. And they will never allow Syria to fall to the hands of the United States, Israel or extremist groups. They will never allow it. You may ask how will this happen. The details will come later," Nasrallah said.
Sectarian strife has been a factor in the civil strife.
The majority of the country is Sunni Muslim; al-Assad's family is Alawite, an offshoot of Shiite Islam.
The Hezbollah leader expressed concern that some Sunni extremists may try to destroy holy Shiite shrines in Syria. While such shrines are highly revered by Shiites, a number of Sunni radicals believe they amount to sacrilege.
One of the most notable shrines is the Sayida Zeinab, which Shiites believe houses the tomb of the Prophet Mohammed's granddaughter, Zeinab. It's located about 10 kilometers (6 miles) south of the Syrian capital.
"Some extremist groups plan to execute their threats with regards to (Sayida Zeinab), and if such a crime were to take place, it will carry with it grave consequences," Nasrallah said.
But the Syrian National Coalition, an umbrella opposition group, dismissed the Hezbollah leader's claims.
"Only the Syrians are responsible to defend and protect their cities and their holy shrines. We don't need the regime militias or Hezbollah fighters to tell us what our duty is," the council said in a statement. "This is our land, our country and our future."
Russia says ships ready to evacuate citizens
The commander of a group of Russian warships said his forces are prepared to evacuate Russian citizens from Syria.
Captain Alexei Komarov said ships due to participate in military exercises off the Syrian coast are standing ready, according to the state-run news agency Itar-Tass.
At least one ship, the Azov, is ported in the Israeli coastal city of Haifa.
There are between 35,000 and 40,000 Russian families in Syria, the Syrian Ambassador to Russia, Riyad Haddad, said last week. He added that only about 100 families have returned to Russia since the start of the civil war. They were relocated in January after being bused out of the country.
A large-scale evacuation from the Syrian coast would be a shift in confidence for Moscow, which has been a staunch ally of Damascus and the al-Assad government.
The Russian military maintains a small base and uses the Syrian port of Tartus for resupply missions.
CNN's Holly Yan contributed to this report.