HONG KONG—The first person to be tried under Hong Kong’s sweeping national security law was found guilty of secessionism and terrorism on Tuesday in a ruling closely watched for indications of how the law will be applied as China tightens its grip on the city long known for its freedoms. Tong Ying-kit was charged with inciting secession and terrorism for driving his motorcycle into a group of police officers last year while carrying a flag bearing the banned protest slogan “Liberate Hong Kong, revolution of our times.” Rights groups condemned his conviction, and many are bracing for further such trials since more than 100 people have been arrested under the legislation—part of Beijing’s increasing crackdown on dissent in Hong Kong following months of anti-government protests in 2019. Tong, a 24-year-old restaurant worker, pleaded not guilty to the charges, arguing the slogan itself does not call for secession. He now faces …