If not looking for natural resources in Europe, China has for years invested heavily across the bloc, its largest trading partner. Yet now concerns are rising that Beijing is using its economic clout for political leverage.

Mr. Douzinas said China had never explicitly asked Greece for support on the human rights vote or on other sensitive issues, though he and other Greek officials acknowledge that explicit requests are not necessary.

“If you’re down and someone slaps you and someone else gives you an alm,” Mr. Douzinas said, “when you can do something in return, who will you help, the one who helped you or the one who slapped you?”

The Trump administration, recognizing it has a geopolitical and economic challenger, recently intervened to help lift an American deal over a Chinese competitor — and the Greeks seemed happy to play one power off the other.

E.U. officials are concerned that China is buying silence on human rights issues and undermining the bloc’s ability to speak with one voice. Analysts say China targets smaller countries in need of cash, among them Spain, Portugal and others that suffered in the financial crisis. Hungary, where China is pledging to spend billions on a railway, also blocked the E.U. statement on the South China Sea.

Many analysts have noted that Greece’s human rights veto came as Prime Minister Alexis Tsipras returned from a summit meeting in Beijing in May, where he signed billions of euros’ worth of new investment memorandums with Chinese companies.