Dong obtained Yu's phone number when they met last month while Yu was delivering donated clothing and food.
"He is a petitioner from northeast China's Liaoning province and has eked out a living by collecting and selling waste in Beijing for more than 10 years," Yu said.
Yu condemned the thieves' acts as "ferocious."
"I don't care if the men in uniform were police officers or city administrators. They seriously harmed these people."
The incident has been widely discussed online. Many have voiced sympathy for the vagrants and anger at the chengguan.
"The homeless never did any harm to the society and deserve to be treated fairly and with respect," wrote "Miss Tan Tan" on Sina Weibo, a popular microblogging site.
Spring Festival draws near, immigrant workers have begun to go home, leading a travel peak at the railway station.