In another setback for the regime, the defection of the military police chief came as military pressure builds on the regime, with government bases falling to rebel assault near the capital Damascus and elsewhere across the country.
The defector al-Shallal appeared in a video aired on Al-Arabiya TV late Tuesday saying he is joining "the people's revolution."
Dozens of generals have defected since Syria's crisis began in March 2011. In July, Brig. Gen. Manaf Tlass was the first member of Assad's inner circle to break ranks and join the opposition.
Al-Shallal is one of the most senior and held a top post at the time that he left. He said in the video that the "army has derailed from its basic mission of protecting the people and it has become a gang for killing and destruction." He accused the military of "destroying cities and villages and committing massacres against our innocent people who came out to demand freedom."
Thousands of Syrian soldiers have defected over the past 21 months and many of them are now fighting against government forces. Many have cited attacks on civilians as the reason they switched sides. Anti-regime activists estimate more than 40,000 have died in the past 21 months.
In violence on Wednesday, the Britain-based Syrian Observatory for Human Rights said government shelling in the northeastern province of Raqqa killed at least 20 people, including eight children, three women and nine others. An agricultural area near the village of Qahtaniyeh was hit by the shelling.
An amateur video showed the bodies of a dozen people including children lying in a row inside a room. Some of them had blood on their clothes, while weeping could be heard in the background.
The videos appeared genuine and corresponded to other AP reporting on the events depicted.