Many were not allowed to leave and, in fact, were herded back into the squalor and danger of the city when they tried to walk to Gretna. There’s a first-hand account here; excerpt:

As we approached the bridge, armed Gretna sheriffs formed a line across the foot of the bridge. Before we were close enough to speak, they began firing their weapons over our heads. This sent the crowd fleeing in various directions. As the crowd scattered and dissipated, a few of us inched forward and managed to engage some of the sheriffs in conversation. We told them of our conversation with the police commander and of the commander’s assurances. The sheriffs informed us there were no buses waiting. The commander had lied to us to get us to move. We questioned why we couldn’t cross the bridge anyway, especially as there was little traffic on the 6-lane highway. They responded that the West Bank was not going to become New Orleans and there would be no Superdomes in their City.

See other coverage, via the first linke, at SF Chron, UPI/Moonie/Right-Wing Times (!), and St Louis Post-Dispatch.