The International Centre for Prison Studies at King's College, London, said on Monday that the United States detains 714 prisoners per 100,000 people.
The US has topped the centre's list since 2000.
"The US has managed in 200 years to reach two million prisoners," said Anton Shelupanov, research associate at the International Centre for Prison Studies. "It's a very concerning rate of growth."
The US total represents 22% of the nine million prisoners reportedly held throughout the world, he added.
Shelupanov told Aljazeera.net that the growth in the number of prisoners in US jails is down to the US's tough penal policy.
The US government says putting criminals in jail is working and recent figures have shown a fall in the violent crime and murder rate.
The George Bush administration also says tough penal policies enjoy wide public support.
However, critics say alternatives to incarcerating people are being ignored, and many prisoners languishing in jails are not dangerous.
"The US has managed in 200 years to reach two million prisoners. It's a very concerning rate of growth"
International Centre for Prison Studies
They also say that people from minority social groups - such as African Americans and Hispanics - are far more likely to end up in prison.
Moreover, human rights groups have often criticised conditions in US jails.
New York-based Human Rights Watch says that "wanton staff brutality and degrading treatment of inmates occur across the country with distressing frequency".
Russia, which has 786,900 inmates, or 550 per 100,000 people, came second in King's College's survey.
Notably high rates of prisoners in European countries include Belarus, with 532 inmates per 100,000 people, and Ukraine with 416 per 100,000.
High-ranking Asian countries include Turkmenistan, at about 489 per 100,000 people according to the United Nations in 2000, and the Maldive Islands, which jails 414 per 100,000 people.
George Bush says tough US penal
policies are working
South Africa is the leading African jailing country with 186,739 of its 44 million people in prison, or 413 per 100,000 people.
Suriname had the top rate in South America at 437 per 100,000 people.
The International Centre for Prison Studies compiles data from a variety of sources, mostly from prison administrations from each nation.
Data for the survey was mainly from mid-2002 or later, the centre said.