Joshua Wong, who led the 2014 pro-democracy protests in Hong Kong, was detained on arrival at Bangkok International Airport, BBC reported.
He was invited by Thai student activist Netiwit Chotipatpaisal to address students at Chulalongkorn University at Bangkok to mark the 40th anniversary of a students massacre in 1976.
Netiwit had told the South China Morning Post newspaper ahead of the visit that it could inspire Thai students to speak against the military leadership.
But when he arrived in Bangkok in the early hours of Wednesday, Wong was stopped by police and immigration officials.
Netiwit said on Facebook that police did not allow him to speak to Wong and told him that they had received a letter from China regarding the visit. But, he did not give proof of his claim.
The Thai government has denied giving any order for Wong's detention.
The military leaders, in power since a 2014 coup, said immigration officials had made the decision to bar him, although no reason for doing so was given.
China's Foreign Ministry said it had noted the reports and respected Thailand's immigration control.
Wong, 19, was at the heart of the mass demonstrations against Chinese influence, known as the Umbrella Movement, which brought parts of Hong Kong to a standstill for nearly three months.
The protestors were calling for free elections for Hong Kong's leadership, without influence from Beijing. The protests ultimately failed to achieve their goal, but several of their leaders have since entered politics.
A Thai Foreign Ministry spokesman said entry into Thailand "has to be in line with the relevant immigration laws and regulations".
Nathan Law, a fellow Hong Kong protest leader who has since been elected to parliament as Demosisto's first MP, also blamed China for the detention.
"China is worried that Wong, an important leader in the Occupy movement, would bring his influence to other countries," he told a Hong Kong radio station.
The rights groups called for Wong's immediate release. Human Rights Watch said his detention "sadly suggests that Bangkok is willing to do Beijing's bidding".
In May 2015, Wong was denied entry to Malaysia where he had been set to take part in talks about democracy in China. He was immediately returned to Hong Kong.
