Protesters, their faces mostly covered with scarves or masks, chanted "No to Chinese embassy" and waved flags reading "Free Hong Kong. Revolution now".
Others held up placards with slogans such as "MI5 warned. Labour kneeled", referring to the UK's domestic intelligence agency and Prime Minister Keir Starmer's ruling party.
Others read: "CCP (Chinese Communist Party) is watching you. Stop the mega embassy."
China has for several years been trying to relocate its embassy, currently in the British capital's upmarket Marylebone district, to the sprawling historic site in the shadow of the Tower of London.
The move has sparked fierce opposition from nearby residents, rights groups and critics of China's ruling Communist Party.
The latest protest came ahead of an expected decision this week.
Benedict Rogers, head of the human rights group Hong Kong Watch said if it got the go-ahead it was "highly likely" that the site "will be used for espionage", citing the sensitive underground communications cables close to the site.
He said China had already been "carrying out a campaign of transnational repression against different diaspora communities" and other critics and predicted that that would "increase and intensify".
Beijing 'operations base' -
A protester who gave his name only as Brandon, for fear of reprisals, said the plans raised a "lot of concerns".
The 23-year-old bank employee, originally from Hong Kong but now living near Manchester in northwestern England, said many Hong Kongers had moved to the UK "to avoid authoritarian rule in China".
But they now found there could be an embassy in London serving as an "operations base" for Beijing.
"I don't think it's good for anyone except the Chinese government," he said.
Another demonstrator, who did not to give her name, called on Starmer to "step back and stop it (the plan) because there is a a high risk to the national security of the UK, not only Hong Kongers".
The 60-year-old warehouse worker, also originally from Hong Kong and now living in Manchester, said the embassy would be a "spy centre not only to watch the UK but the whole of Europe".
Speakers at the rally throwing their weight behind the campaign to stop the embassy included Kemi Badenoch, leader of the main opposition Conservative Party.
British MPs voiced major security concerns earlier this week after a leading daily reported the site would house 208 secret rooms, including a "hidden chamber".
The Daily Telegraph said it had obtained unredacted plans for the vast new building which would stand on the historical site of the former Royal Mint.
It showed that Beijing reportedly plans to construct a single "concealed chamber" among "secret rooms" underneath the embassy which would be placed alongside the underground communications cables.
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