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In this photo released by the Syrian official news agency SANA, Arab league observers, right, icheck with a Syrian injured man who wounded during an explosion at Midan neighborhood in Damascus, Syria, on Friday Jan. 6, 2012. An explosion ripped through a police bus in the center of Syria's capital Friday, killing at least 10 people and possibly 25 in an attack authorities blamed on a suicide bomber, an official and state-run TV said.

BEIRUT — Activists say Syrian troops have opened fire on demonstrators holding sit-ins in two parts of the country, killing one and wounding at least 20.

The Local Coordination Committees group says the troops fired late Friday upon scores of protesters who been camped out in the central square of the northern town of Saraqeb for eight days. The British-based Syrian Observatory for Human Rights reports at least 20 were wounded.

Both groups also report attacks by troops on Saturday on another sit-in in the restive central city of Homs, during which at least one person was killed.

The crackdowns follow a blast in the capital Damascus that killed 25 people on Friday. Syria's interior ministry said after the blast that it would respond to security threats with an "iron fist."