Queues of customers waiting to place orders at fast-food chains in the Ratchaprasong and Pathumwan areas were extremely long. Nearly all customers were protesters.
McDonald's, KFC, Starbucks, food courts and street food vendors around the major protest areas were very busy on Monday.
"We expected sales at quick-service restaurant chains and food restaurants in our MBK shopping centre to double from the normal situation," said Sakchai Kengkijkosol, managing director for retail business of MBK Plc.
Many protesters rushed to have lunch at MBK's food court, extending the peak hours from 11am to 3pm, two hours later than normal.
Mr Sakchai said the number of visitors to MBK on Monday was about 89,000, on a par with a weekend.
Paisarn Aowsathaporn, executive vice-president for food business at Oishi, said the company enjoyed big sales at Oishi Grand at Siam Center and at its Shabushi outlets at Central shopping complex on Chaeng Watthana Road and CentralWorld at Ratchaprasong.
Normally, Monday has the lowest food sales of the week, but sales yesterday climbed to the same peak level as Saturday.
Similarly, Swensen's (Thai) Co operations director Suwannee Chansrigaydorn reported rising sales at its branches at Siam Paragon, Siam Center, Terminal 21, Century Complex, Centre One and The Emporium.
The company moved staff from other branches in Bangkok to help at the six busy ice-cream outlets.
Phattaraporn Phenpraphat, vice-president for marketing at Central Food Retail, said its food courts at CentralWorld and CentralPlaza Ladprao enjoyed brisk sales and some rice and curry stalls had to stop services early as they ran out of ingredients.
In the Asok area, Terminal 21 was packed with protesters identified by their whistles and T-shirts, with some carrying signs in both Thai and English to express their anger towards the government. Some even used the first floor of the shopping mall, which is connected to the Asok skytrain station, to sit and rest.
On the streets, food and drink vendors welcomed the rallies as a boost to their income.
A coffee street vendor on Phahon Yothin Road said he earned more than double his usual income from people who joined the rallies, though regular customers were fewer in number.
A black jelly juice vendor moved from her regular site near the Ari skytrain to the Mor Chit skytrain hoping to cash in on the rallies.