HONG KONG—A trial began Monday for Hong Kong media tycoon Jimmy Lai and seven other pro-democracy activists charged over their roles in an unauthorized Tiananmen vigil last year, amid a clampdown on political dissent in Hong Kong. Lai and the seven others, which includes Lee Cheuk-yan, the former chairman of the now-defunct Hong Kong Alliance in Support of Patriotic Democratic Movements of China, face charges that include organizing, participating, and inciting others to take part in the unauthorized candlelight vigil commemorating the bloody 1989 clampdown on protesters in Beijing’s Tiananmen Square. Lee and four other defendants pleaded guilty to the incitement charge as the trial began. Former Democratic Party chairman Wu Chi-wai pleaded guilty to taking part in the unauthorized vigil, while Lee also pleaded guilty to organizing it. They will enter mitigation pleas Nov. 12 before they are sentenced. Lai, the founder of the now-defunct pro-democracy newspaper Apple Daily, …