The Guardian reported that throughout the morning white-clad people streamed through Waverley Station and walked to a city park.
"We have had enough political spin, promises and downright lies," Postlethwaite said in a speech.
The demonstration is one of a series of events planned for Edinburgh in advance of the G8 meeting, which begins Wednesday in nearby Gleneagles.
At 3 p.m. local time, the marchers held a two-minute silence, and then circled the city center holding hands, the New York Times reported.
Mike Dewar, a retired firefighter, told the Times G8 leaders "seem to be immune in their ivory towers. We don't all need money. Just a decent way to earn a living."