BEIRUT — 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.