A meal of dog meat and lychee fruit on the summer solstice, which fell on Friday this year, is considered good for health by residents in the southern city of Yulin, China's official Xinhua news agency reported.
Activist Hong Bin calls the event "inhumane." He says he is not against eating meat, "I just think people should behave in a more humane way."
Poison may have been used to kill some of the canines, he added, making their meat harmful to humans.
Several organizations released a joint statement Tuesday calling for an end to the festival on the grounds it had created a black market for the stealing and butchering of dogs.
Yulin residents say the festival is a longstanding tradition and it isn't against the law.
The deputy mayor of Yulin, Mang Changxue, said it would be difficult for the city to stop the event but officials can crack down on dog theft and take measures to ensure the meat is safe to eat. Still, public disgust over the festival appears to be growing, the Los Angeles Times reported.
The response on social media "has been terrific," said Grace Gabriel, Asia regional director for the International Fund for Animal Welfare. She noted more than 100,000 people had signed an online petition calling for an end to dog meat festivals.
The government has stopped issuing permits for dog-slaughtering facilities and is not renewing existing permits, said Song Jinzhang of the China Small Animal Protection Association.