| Turn off everything electronic or motorized until you get stable power. | A few incandescent lights won't be a problem. If in doubt, open the main | service breaker.
[elaborating ...]
Electronic devices that can automatically handle the full voltage range of 100-240 volts, when plugged into a 240 volt outlet, will be OK if the voltage at that point is at least 100 volts. It may well be in certain cases. However, the crisis time is not the time to rig up something to connect a computer to a 240 volt circuit.
A common cause of this kind of brownout is the loss of one phase of the three phases supplying the primary of a delta-wye transformer. Without one of the phases, the two that remain are powering one winding in the normal way, but powering the other two windings of the delta loop as if they were wired in series. The effect is to split the voltage to each of those two windings, with the proportion depending on the load balance, which will typically be fairly near balanced in normal circumstances. So 2 out of 3 of the secondaries will have about half the voltage.
Even at half the voltage, enough current could be drawn through a motor winding of a stalled motor to create sufficient overheating to damage it. So definitely unplug appliances with motors, or switch off the breakers.
| Are you sure a neighbor isn't inadvertently running a generator back | into the utility lines? That could kill a lineman.
That is one possible cause of a brownout. Even though it is quite rare, since the load would typically cause the generator to trip or stall, it is very possible, and very serious if it is happening, due to the risk it poses to the linemen. Do check all your neighbors to be sure everyone is OK, and while that, see if anyone is trying to run a generator without the essential transfer switch.
Equipment that intentionally feeds power back to the grid should have the ability to cut off when the grid is down. But something defective could be trying this as well. It would have to fail both at detecting power and fail at tripping on an overload ... but don't rule it out because of seriousness of the danger.
Never touch any utility wires, even if the power is off, even if you are are handling the insulated part, and even if you are sure they are just telephone or cable wires. Damaged power lines can introduce primary distribution voltages into any other wiring, and the insulation intended for lower voltages will not protect you.