The boy was detained by the Chinese authorities just three days after the Dalai Lama declared him to be the reincarnated Panchen Lama, the second most important figure in Tibetan Buddhism.
China has divulged little information about Gedhun Choekyi Nyima or his whereabouts - it says this is necessary to protect him from being "kidnapped by separatists".
Banned from meeting foreign visitors, China says he has attended school and is now leading a normal life in China.
Wangdue Tsering, press spokesman for the Office of Tibet in London, told the BBC that events were taking place today across the world to mark the 20th anniversary of Gendhun Choekyi Nyima's disappearance, including a candle-lit vigil outside the Chinese embassy here.
"We are appealing to the world community to help us find out where the Panchen Lama is," Tsering told the BBC.
"It's 20 years since he disappeared and we don't know where he is, where his family is and how he is. We want the Chinese authorities to give some information," Tsering said.
In 1959, after a failed anti-Chinese uprising, the 14th Dalai Lama fled Tibet and established a government in exile in India.
Beijing views the 79-year-old Dalai Lama as a separatist, although the Nobel laureate has repeatedly stated that his goal is for Tibetan autonomy rather than independence.
