The World Health Organization (WHO) said it has identified counterfeit versions of India's primary Covid vaccine, Covishield.
The doses were seized by authorities in India and Africa between July and August, a WHO statement said.
It also said the vaccine's maker, Serum Institute of India, confirmed that the doses were fake.
The WHO warned that fake vaccines "pose a serious risk to global public health".
It called for their removal from circulation.
There has been no official statement by the Indian government, but local reports said the country's health ministry was investigating the matter.
"Although we have a strong system to prevent such cases, with this development, the only thing we want to ensure is that no Indian received a fake vaccine," an unidentified health official told the Mint news website.
Covishield is the Indian-made version of AstraZeneca's jab and is the most widely used vaccine in India with more than 486 million doses administered so far.
Serum had supplied millions of Covishield vaccines to countries in Asia, Africa and South America - as part of deals that were inked with various governments and the global Covax scheme for poorer countries.
India also sent Covishield doses to some of its neighbours as part of Prime Minister Narendra Modi's "vaccine diplomacy".
But in the wake of a devastating second Covid wave in April and May, the government decided to accelerate the vaccine drive and banned exports.
Serum has since maintained that their priority remains India's own needs - and they may not export again until the end of this year.
India, which is the second worst-affected country in the world, aims to vaccinate all its people by the end of this year.
About 13% of the population has been fully vaccinated since the beginning of the drive in January.