This is called a swarm, and the bees are not dangerous and are rarely defensive at this time because they have no home to defend. When a hive swarms, the queen bee and roughly half of the worker bees are moving to a new location to begin another beehive. They pile up and send out scout bees to find the ideal new home.
The scary-looking swarm will usually disappear in a day. If bees have swarmed in an area where children or pets can knock or disturb the swarm, please find a local beekeeper to remove them rather than an exterminator!
(Note: If you are in an area that has Africanized honeybees, then they could be dangerous.)