Last Updated 3:42 p.m. ET
(AP) DAMASCUS, Syria - 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 yards 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 Qaeda 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 Qaeda's involvement could further fuel the sectarian tensions that the uprising has already stoked. Al Qaeda'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.