They help move families to higher ground and rescue what they can of their household goods and belongings.
Meanwhile, a delegation sent afterward to appeal to provincial authorities for help has not returned, a local source says.
Novices aged 15 and younger must be enrolled instead in government-run schools, authorities say.
Many observe the day in private, or hold picnics in the countryside away from public view, sources say.
The Tibetan spiritual leader, who fled into exile in India in 1959 to escape Chinese rule, turns 83 on Friday.
They are taken into custody amid a province-wide crackdown on 'separatist influence,' a local source says.
Activists warn that the move will leave a power vacuum in international human rights issues that China will fill on its own terms.
Heavy fines are threatened for those refusing to take part, local sources say.
Dorjee Gyantsan had gathered information on refugees' names and political views and will now serve 22 months in jail.
No explanation is given for Lobsang Tenzin's early release from a 10-year prison term for 'aiding and abetting' a self-immolation protest.
Pema Gyatso, a student at Lanzhou's Northwest University for Nationalities, had previously been warned that his writings were politically sensitive.
Government employees, schoolchildren, and retirees are ordered not to take part, a local source says.
Fewer teachers able to teach in both Mandarin and Tibetan are being hired, disadvantaging Tibetans, they say.
Observers call for his release, saying he followed Chinese law in promoting his mother tongue.
Dashar, a resident of Sershul county in Sichuan's Kardze prefecture, had been jailed for 10 years for his role in protests in Lhasa in 2008.
They are told not to visit monasteries or attend religious gatherings during the Buddhist holy month of Saga Dawa.
Chinese authorities fear the spread of 'separatist' sentiment inspired by exiled spiritual leader the Dalai Lama.
Already partially fenced for farming, the grazing land is now being taken for development, a local source says.
The representative of the Trump administration to India calls the spiritual leader 'inspirational,' voices hope he may soon visit the US.
Police have now blocked communications in the area to keep more news from getting out.
Woechung Gyatso was taken into custody after participating in a Chinese-led workshop on laws regulating internet use.
Meetings with a Chinese embassy official in Poland and a journalist in Sweden were unconnected with intelligence work, he says.
Local Tibetans lose land they have farmed for generations, with compensation paid at below-market rates.
Lhamo Dolkar, 60, came under suspicion after she visited a political prisoner in jail some six years before her detention.
The suspect is believed to have been paid by Chinese agents for information about the diaspora.
It was Labrang monk Jinpa Gyatso's second period of incommunicado detention in less than a month.
Listed offenses include environmental activism and the promotion of Tibet's national culture and language.
Lobsang Sangye was sentenced in a closed trial after seven months of incommunicado detention.
Police take a village leader into custody after he refuses to sign his approval for the project, a local source says.
A former leader of Tibetan student protests, he was sentenced for his writings criticizing Chinese policies in Tibetan areas.