Protesters faced down deadly gunfire from Syrian security forces on Friday as they returned to the streets of several restive cities, as Anthony Shadid reports. Videos captured their marches, as well as some of the violence that left dozens dead, the latest in the six-month crackdown that has left more than 2,600 dead.

In the central city of Homs, activists reported deaths on Friday and posted video online depicting chaotic scenes of large crowds fleeing heavy gunfire.

The voice in the video below, which was posted to an activist Facebook page focused on events in Homs, shouts the date — Sept. 16, 2011 — and that he is in the Khalidiya area of Homs.

“Shots have been fired!” he can be heard saying. “Many shots.”

Another jolting video captured what appeared to be government forces positioned on a road in Homs. Two men enter the frame toward the end of the clip before noticing the vehicles and appearing to immediately move away.

In an e-mail, the anonymous activists who maintain the Facebook page where the videos were posted confirmed that they were from Friday in Homs. They noted that the videos showed the police shooting unarmed people.

In another video said to have been from Friday in Dara’a, security forces used tear gas to prevent a chanting crowd from exiting a mosque and marching through the city.

Another, uploaded by to the same account at roughly the same time on Friday, shows Syrian security forces surrounding the mosque.

As violence erupted in the street, Robert S. Ford, the American ambassador in Damascus, took to Facebook for the second time in two weeks to respond to critics who have challenged his blunt assessment of Syrian President Bashar al-Assad’s brutal crackdown.

Mr. Ford, as he did last week, thanked the “thoughtful criticisms and questions raised here,” and said they deserved a response, which he gave to individual commenters by name:

Ghazal Mahran rightly points out that the “mission of any army in the world is to maintain security and stability.” No one would criticize the Syrian security forces for maintaining security and stability if they did so in a manner that respected human rights and were held security accountable for human rights violations.

But he added: