The Tibetan government in exile has estimated that 1.2 million Tibetans have died since the start of the occupation.
Two protesters from the Free Tibet Campaign climbed onto a balcony of the central London embassy and flew the Tibetan flag in a symbolic gesture on Saturday. One person was arrested.
The protest comes a week ahead of talks in China - involving western companies - on ways of developing Tibet's natural resources.
"The occupation of Tibet is a half-century-old moral outrage, yet China has effectively silenced the international community," said Alison Reynolds, Director of Free Tibet Campaign.
"Now China is entering a new phase, consolidating its position by asking Western companies like BP to finance the exploitation of Tibet's natural resources.
"This does not bode well for the next 50 years. When is the West going to stand up to China?"
On 7 October 1950, 40,000 soldiers from the People's Liberation Army crossed the upper Yangtse river and entered Tibet, ending decades of independence.
The Tibetan government in exile has estimated that 1.2 million Tibetans have died since the start of the occupation.
China is stepping up plans to extract Tibet's resources, and is seeking international finance to assist in this.
Free Tibet campaigners fear this will cause the migration into Tibet of large numbers of Han Chinese.
This, they say, "continues the policy of reducing Tibetans to a minority in their own country and eroding their unique cultural identity".
The Free Tibet Campaign also says that China is engaged in a further crackdown on anything that symbolises an independent Tibet, conducting late night house-to-house searches for photographs of the Dalai Lama.
China has announced it intends to "mark the 50th anniversary of the peaceful liberation" of Tibet next year - Chinese troops entered Lhasa in October 1951.
It routinely denies human rights abuses in Tibet and points to the construction of roads, factories and hospitals as evidence of its "benevolent" rule.
The Free Tibet Campaign group has said it will be running a major campaign between now and October 2001 to commemorate "half a century of injustice and oppression".
This will focus attention on "the West's complicity in the occupation, and the continued impact on individuals".
Deputy Prime Minister John Prescott will be in China, for Britain in China Week, which starts on 16 October.