The journalists, three from Now TV, two from Cable TV and one from Reuters news agency, were held for nearly six hours, one of them said.
"We were told we were being formally summoned for allegedly causing public disorder," said Lam Kin-shing, the Guangzhou correspondent for Cable TV.
The rare protest was the first of its kind to mobilise Cantonese speakers in Hong Kong and Guangdong in defence of their dialect, which they say has been increasingly marginalised on the mainland.
More than a hundred people had gathered in protest, said writer Ye Du, who was among them. They were cheered and joined by dozens of onlookers before police ordered them to leave. Hundreds more gathered outside the park, unable to get in whilst police were clearing it.