BEIRUT (AP) ― An explosion struck near a Syrian government security building in the northern city of Aleppo Sunday, while a harsh security crackdown prevented opposition rallies marking one year since the first nationwide protests of the uprising against President Bashar Assad.

The Syrian state news agency called the Aleppo explosion a “terrorist bombing’’ and said one policeman and one female civilian were killed, while 30 were injured. It was the second attack in two days on regime strongholds.

Three suicide bombings in the capital Damascus on Saturday killed 27 people. Two of them also targeted government security buildings and the regime the opposition, which it claims is made up of “terrorist’’ groups carrying out a foreign conspiracy.

Aleppo and Damascus, Syria’s two largest cities, have been struck by a number of suicide bombings since December. Both are critical centers of support for Assad and have remained relatively insulated from the unrest shaking much of the country for the past year.

No one has claimed responsibility for any of the weekend attacks.

Mohammed Saeed, an Aleppo resident, said a car bomb exploded early afternoon about 200 meters from the Political Security Directorate. Security forces started shooting in the air and cordoned off the area to prevent people from approaching.

“It was a strong explosion. It shook parts of the city,” Saeed said, citing nearby residents. “White smoke was billowing from the area.”

The explosion did not seriously damage the security building, he said. At that time of day the city’s central Suleimaniyeh neighborhood is usually crowded with people, especially on Sunday, the first day of Syria’s workweek.The neighborhood has a large Christian population, Saeed said.

The string of large-scale bombings near government security buildings in Damascus and Aleppo that have added a mysterious element to the anti-government revolt. After other similar attacks, U.S. officials suggested al-Qaida militants may be joining the fray.

A previously unknown Islamist group calling itself Al-Nusra Front to Protect the Levant claimed responsibility for previous attacks in a video posted online, saying it carried them out “to avenge the people of Homs.” Homs is an opposition stronghold in central Syria that has been hard hit in the government crackdown.

Al-Qaida’s involvement could further fuel the sectarian tensions that the uprising has already stoked. Al-Qaida’s supporters are largely Sunni Muslim extremists. Syria’s military and political leadership is stacked heavily with members of the minority Alawite sect, an offshoot of Shiite Islam to which Assad and the ruling elite belong. The Alawite leaders of Syria are closely allied with Shiite Iran.

Sunnis are the majority in the country of 22 million and make up the backbone of the opposition.

The last major suicide bombing in Aleppo was on Feb. 10, when twin blasts struck security compounds, killing 28 people. Damascus has seen a half dozen suicide bombings since December, most hitting intelligence and security buildings.

Many activists consider March 18, 2011, the start of the anti-Assad uprising. Thousands took to the streets in cities across Syria on that day, and security forces killed marchers in the southern city of Daraa.

Since then, Assad’s security forces have sought to crush all signs of dissent, and protest and international condemnation have spread. Many in the opposition have taken up arms to defend themselves and attack government forces as the increasingly militarized conflict has become one of the bloodiest of the Arab Spring.

The U.N. says more than 8,000 people have been killed.

Jordanian Information Minister Rakan Majali said Sunday that attempts by arms dealers to smuggle weapons into Syria have gone up since the start of the year, but that Jordanian authorities foil them. He said Jordan’s northern border with Syria is being “carefully watched’’ by Jordanian security.

Also Sunday, activists posted videos online of an overpass on a main north-south highway they said was destroyed by opposition fighters near the village of Khirbet Ghazaleh. The videos said the attack sought to block the army from bringing more tanks and military reinforcements into the area.

The Syrian state news agency said an “armed terrorist group’’ blew up the bridge and estimated the damage at more than $5 million.

Activists also reported clashes between Syrian troops and army defectors in the eastern city of Deir el-Zour, where at least three people were killed. The area is close to the border with Iraq and witnessed clashes in the past.

SANA reported later in the day that “armed terrorists’’ shot dead 13 people, including women and children, in the village of Hassiba in the central province of Homs.

A Syrian opposition figure told The Associated Press that the people killed in Hassiba were 14 Shiites shot dead for sectarian reasons. The opposition member spoke on condition of anonymity because of the sensitivity of the matter.

The Britain-based Syrian Observatory for Human Rights said pro-government thugs known as “shabiha’’ quickly broke up a protest in Damascus by hundreds of people.

“They had just started protesting, but then some started chanting ‘The people want to topple the regime’ and the shabiha started beating them,” said Rami Abdul-Rahman, the group’s head.

Prominent opposition writer Mohammed Sayid Rasas was arrested, as were other protest leaders, he said.

The Observatory said at least 16 people were killed throughout Syria Sunday, while the Local Coordination Committees, another activist group, said 29 were killed. The count did not include those killed in Hassiba.

The Syrian government has barred most media from operating in the country, and activist claims could not be independently verified.