Kemi Badenoch led hundreds of protesters in opposition to China's proposed new mega-embassy in central London.
Crowds were warned plain clothes Chinese police officers were among those gathered at the site of the former Royal Mint.
It was the last chance for demonstrators to voice their concerns ahead of the government's decision on the controversial planning application on Tuesday.
Chants of "mega embassy, mega mistake", "don't bow to China" and "no mega embassy" rang out, while protesters raised their middle fingers towards the huge site.
The location between the financial districts of the City of London and Canary Wharf has raised security concerns the embassy will be used as a spy hub, while dissidents fear it will be used as a base for repression.
The Tory leader took to a makeshift stage next to the site to assure protesters: "I stand with you".
"I tell you, I will always fight for freedom. I am calling on the government to stop this decision," Ms Badenoch said.
"We do not want a country that spies on our MPs having a super embassy right here, and I will do everything in my power to continue speaking on your behalf."
Reform UK MP Danny Kruger, Liberal Democrat MP Bobby Dean and Conservative MP Sir Iain Duncan Smith were also among the speakers.
Sir Iain urged the government to "wake up" to the threat from China and said he was not reassured by the apparent lack of intervention from the UK security services.
"The reality is that many in the security services know what the threat is, but they're also kiboshed by the Foreign Office and the government," he told Sky News.
Some in the crowd carried placards featuring Sir Keir Starmer in compromising positions with Winnie the Pooh - often used to satirise Chinese president Xi Jinping.
The government is expected to approve the controversial plans on Tuesday - ahead of the prime minister's expected visit to China later this month.
Many in the crowd were members of Chinese, Uighur and Tibetan diasporas in the UK or Hong Kongers exiled because of their pro-democracy activities.
Chloe Cheung, 20, who has a HK$1m (around £100,000) bounty on her head and is subject to an arrest warrant, said she already faces repression in the UK from China.
"It's devastating that I have been fighting for democracy, which is one of the British values," the Committee for Freedom in Hong Kong activist told Sky News.
"I don't have any intention to harm Britain but I have been harassed and threatened every day because of speaking out for democracy."
She said she was "horrified" at the prospect of the new mega embassy, adding: "The worst-case scenario is someone will kidnap me to the embassy."
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China maintains the new embassy is being built to "promote understanding and friendship" with the UK and "develop mutually beneficial co-operation".
A Chinese embassy spokesperson previously told Sky News claims the new embassy poses a potential security risk to the UK are "completely groundless and malicious slander, and we firmly oppose it".
A Foreign Office spokesperson said: "It is a normal feature of diplomatic relations for countries to maintain embassies -we have embassies worldwide, including in Beijing.
"It goes without saying that national security has been a priority through this process. In terms of process, the decision will be made independently and the Ministry of Housing, Communities and Local Government has set out that a decision will be made on or by 20 January."
Residents' groups have already indicated they plan to challenge any decision to approve planning permission in the courts, so these demonstrations are likely to become a common sight.
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