Is that believable? Sure, but locking arms is also a common thing during protests. The road was already blocked off and cars weren't going through. Also, they fired CS into crowds of protesters blocks away, claiming that they think the protesters are coordinating by secure communications and instantaneously turning violent across the city. Most of it seems like bullshit, and even if it's all true, locking arms on one side of a closed street is not at all unusual. They should have arrested the one guy hitting the glass and let the protest continue.
By the way, they used this event to justify the curfews that haven't been raised since then.
That doesn't make sense, unless they wanted the driver and other people in a not airtight emergency vehicle to end up in the gas cloud as well...
EDIT: I will also note that this, among other events, was used to justify calling for a curfew in town, and today, the mayor and police chief have condemned their own police officers for deploying tear gas on peaceful protests prior to the start of the curfews last night, and then putting forward a false justification (admitted by the police chief to have been a false justification, to be clear) for doing so.