Police have not ruled out further arrests.
"You could break the law without intending to."
It has been criticised for undermining freedoms guaranteed under a 'one country, two systems' formula agreed with China.
He was carrying a "liberate Hong Kong sign" as he drove into police at a protest.
"China strongly condemns this and reserves the right of further reactions."
"I'm scared of going to jail but for justice I have to come out today, I have to stand up."
Hundreds of protesters gathered downtown for an annual rally to mark the 23rd anniversary of the colony's handover to China.
The law has many concerned.
Beijing is expected to set up a national security office in Hong Kong to "supervise, guide and support" the city government.