Piers Corbyn has been found guilty of breaching coronavirus restrictions at an anti-lockdown gathering.
The 73-year-old brother of former Labour leader Jeremy Corbyn was arrested when he refused to leave the event in Hyde Park, London, on 16 May.
He was given an absolute discharge after Westminster Magistrates' Court heard he had spent 12 hours in police custody after his arrest.
Addressing supporters outside, Corbyn said it had been "a tremendous result".
District Judge Sam Goozee dismissed a second count of the same charge - linked to a protest on 30 May - after hearing police had issued a fixed penalty notice earlier that day.
Prosecutor David Povall had described Corbyn as a "poster boy for disparate groups" attending both events near Speakers Corner.
He told the court there was no reasonable excuse for "breaching clear and emphatic regulations that were in force at the time".
Corbyn's defence had argued his arrest on 16 May was a "disproportionate and unnecessary" contravention of his right to peaceful protest.
Returning his decision, judge Mr Goozee said Corbyn's actions would have been lawful if lockdown regulations had not been in force at the time.
But their enforcement had been necessary for public health, he said, concluding that police "took a measured response".
"You, however, didn't engage with police - police action in arresting you was necessary and proportionate," he said.
Addressing around two dozen supporters outside the court after the verdict, Corbyn raised his fist in the air and said: "We've had a tremendous result."