Martin Lee, the 81-year-old founder of Hong Kong’s Democratic party, has said there will be more fatalities and protests if authorities try to pass anti-subversion laws – which would outlaw “sedition, subversion and the theft of state secrets” – before the September legislature election.

“It will result in deaths for sure,” he told the Guardian. “The Communist party won’t show any mercy. They have already stated their stance.”

Lee said he had felt guilty seeing some 7,000 people arrested during the months-long anti-government movement, but that his own arrest on Saturday meant he could at last look them in the face. “It was a good thing,” the barrister said.

Lee said his arrest, along with 14 other veteran pro-democracy figures on charges of illegal assembly, and high-profile posturing from China over the past week, were part of Beijing’s wider plan to tighten its control over Hong Kong.

China’s liaison office in Hong Kong last week called for national security legislation and controversially declared it was not bound by a clause in the city’s post-handover mini-constitution, the Basic Law, which bars the Chinese government from interfering in local affairs. Hong Kong is guaranteed “a high degree of autonomy” under the one country two systems principle enshrined in the Basic Law, but the liaison office said this did not mean “complete autonomy”.

“It would come sooner or later. They already said in 2014 they would exercise ‘complete jurisdiction’ in Hong Kong,” said Lee, once trusted by Beijing as one of the drafters of the Basic Law.

A Beijing policy white paper asserted that China had “comprehensive jurisdiction” over Hong Kong and the city’s “high degree of autonomy … comes solely from the authorisation by the central leadership”.

“China has breached the agreement, not only with Hong Kong but with the international community,” he said, referring to the 1984 Sino-British Joint Declaration, a legally binding treaty registered with the United Nations. “The international community has a moral obligation to speak out.”

“They are now completely besieging Hong Kong, they see no need to be courteous,” he said. “They want Hongkongers to be as obedient and subservient as people in any other Chinese city. They want complete control.”

Still, Lee said he would not give up on his fight for democracy, even if he was to go to jail.

“I’m obviously disappointed but this has strengthened my will,” he said. “I won’t give up because we have the moral high ground.”