After two paragliders flew over the circuit trailing a banner protesting against race sponsor Shell's Arctic drilling plans, four activists from Greenpeace Belgium scaled the roof of the main grandstand in the hour before the start.
As local police struggled to find a way of stopping the protest, the protestors unfolded a 20-metre long banner that read: "Arctic oil? Shell no!"
In a statement issued following the protest, Greenpeace international executive director Kumi Naidoo said: "I was a fan of Grand Prix racing when I was growing up, but I am not a fan of what Shell is doing in the Arctic.
"Right now we are in the race of our lives against Shell, a company that sees the melting of the Arctic as a business opportunity, rather than a warning. Every driver and F1 fan knows that oil on the tracks spells disaster; an oil spill in the Arctic would be catastrophic.
"We hope that when they've heard about what Shell is up to they'll join the almost four-million-strong movement to save the Arctic."
The Belgian Grand Prix went ahead without any delay despite the events on the main grandstand.