Syrian anti-government protesters hang a banner in Arabic that reads "Thank you al-Jazeera and Shaam News Network" during a rally in the central city of Hama that was one of many such events across Syria on July 15, 2011. (AP)

As many as 19 protesters were killed after security forces reportedly shot at the crowds. Hundreds of thousands of people took to the streets Friday in the biggest protests since mid-March. The day was dubbed “Freedom of Prisoners Day,” in recognition of those who have been detained in the protests.

“We are in Midan and they are firing tear gas on us, people are chanting,” a witness told al-Jazeera from Damascus, one of at least five cities with Friday protests.

Reports streamed in on Twitter and on YouTube. Watch video from the city of Deraa, as protesters run from gunfire:

This video shows protesters making an organic RPG launcher:

Syrian president Bashar al-Assad is facing the greatest challenge to 40 years of his Baath Party’s rule.