Yes, I think it was the legal community that was involved in the very first protest because it saw the implications of the bill. I only know that because I got a Facebook invite to participate in that protest (unfortunately I am actually thousands of miles away or I would have gone). Anyway, people were shocked to see all these respected barristers, solicitors and retired judges participating, which opened everyone’s eyes to the danger of the bill.
Sorry, it wasn’t the first, but it did shock Hong-Kongers because it really emphasized this was a serious threat to civil liberties.
From Wikipedia;
June 6 lawyers’ silent march
Thousands of lawyers marched in black against the extradition bill on 6 June 2019.
The legal professionals who had raised concerns over the extradition bill also staged a silent march on 6 June. Wearing in black, the lawyers, legal academics and law students marched from the Court of Final Appeal to the Central Government Offices. Led by Dennis Kwok, Legislative Councillor for the Legal constituency, and also two former Hong Kong Bar Association chairmen, Martin Lee and Denis Chang, the lawyers then stood in front of the government headquarters looking at the building for three minutes silently.[23] Attended by more than 3,000 Hong Kong lawyers, representing around one quarter of the city’s lawyers, it was the fifth, and the largest, protest march held by lawyers in Hong Kong since 1997.[24]
While the lawyers expressed grave reservations about the openness and fairness of the justice system in China, limited access to a lawyer, and the prevalence of torture, Secretary for Security John Lee said the legal sector did not really understand the bill.[24]
https://en.wikipedia.org/wiki/2019_Hong_Kong_anti-extradition_bill_protests