Sham told RTHK that he was having lunch with a friend on Tak Hing Street in Jordan, when the two men came into the restaurant and smashed their weapons on the table.
When the men lunged at him, Sham said his friend tried to stop them but was hit on his left arm three times.
Sham said his friend suffered bruises and was sent to hospital.
Earlier in the day, a group of pro-government people surrounded Sham in what they said was action to “denounce” the front. The group called Sham a rioter and tried to snatch a loudhailer from him.
The group dispersed after police officers intervened.
The Civil Human Rights Front, which organised mass extradition bill rallies over the summer, has been refused permission for another protest on Saturday. It is appealing against the police ban.