I live in Thailand, not in Bangkok though. what I can tell from farang (foreigner) perspective, the two sides red and yellow shirts are both sponsoring "protesters" to capital with free food and other perks. its generally very peaceful and laid back affair, listening concerts and then political speeches. but both sides have hard line that have been provoking opposing side now for months, and yes it has got nasty occasionally. still, so far majority of demonstrators are not angry mob that want to burn everything in its path.
things erupt like this in Thailand every now and then. it was much more serious back in 2006 when Thaksin (then prime minister) was ousted from his position and exiled, by military coup.