Inside the stadium, the lot-drawing process appeared to be going on unaffected.
The protesters are demanding that Prime Minister Yingluck Shinawatra step down. They oppose the polls scheduled for February 2 because Yingluck is seen as sure to win them.
Thailand has been wracked by sometimes violent political conflict since Yingluck's brother, former Prime Minister Thaksin Shinawatra, was toppled by a 2006 military coup. The protesters accuse Yingluck of being a proxy for Thaksin, who lives in self-imposed exile to avoid jail time for a corruption conviction but still wields influence in the country.
Thaksin or his allies have won every election since 2001 thanks to strong support in the north and northeast of the country. His supporters say he is disliked by Bangkok's elite because he has shifted power away from the traditional ruling class, which have strong links to the royal family.
On Wednesday, Yingluck announced a proposal for a national reform council to come up with a compromise to the crisis, but it was rejected by the protesters. They now plan more civil disobedience and street protests in a bid to provoke such chaos that Yingluck will be forced to resign.