Al-Najjar, 73, was arrested in Aleppo city on 4 February for street demonstrations, after he called on Syrians during Friday prayers to fight for change in the country.
Local human rights groups are calling for al-Najjar to be given a fair trial.
Security services pre-emptively summoned more than 10 activists at the beginning of February in an attempt to pressure them not to demonstrate, according to Human Rights Watch.
However, due to heavy rain, the planned protests ended fruitlessly, with almost zero turn-out in the capital city of Damascus.
On Wednesday, the exiled cousin of Syrian President Bashar Al-Assad called on authorities to respond to popular pressure.
"We hope that the leaders in Syria are a little more intelligent than their counterparts in Tunisia and Egypt," Ribal Al-Assad told a press conference in Berlin. "We've called for change; otherwise they'll be the ones to be changed."