US lawmakers have nominated Hong Kong's pro-democracy movement for the Nobel Peace Prize, calling the campaigners a global inspiration as they endure a crackdown by Beijing.

In a letter to the Nobel Committee submitted by the February 1 deadline and released Wednesday, nine lawmakers across party lines cited the estimate that more than two million took to the streets on June 16, 2019.

With Hong Kong having a population of 7.5 million, it amounts to "one of the largest mass protests in history," said the letter, led by Republican Senator Marco Rubio and Democratic Representative Jim McGovern, co-chairs of the Congressional-Executive Commission on China that assesses human rights.

"This prize would honor their bravery and determination that have inspired the world," they wrote.

"We hope that the Nobel Committee will continue to shine a light on those struggling for peace and human rights in China and we believe the pro-democracy movement in Hong Kong is deserving of recognition this year."

The lawmakers said they expected more Hong Kong democracy activists to be convicted in the coming months "for the sole reason of peacefully expressing their political views."

Beijing last year imposed a tough security law that it has used to clamp down on dissent after the unrest in the financial hub, which was promised a separate system before Britain handed the territory back to China in 1997.

Among those who have been rounded up include 24-year-old Joshua Wong, one of the city's most recognizable democracy advocates, who has been charged with subversion.

The lawmakers recommended that the prize go broadly to all who have pushed for human rights and democracy in Hong Kong since 1997.

Elected lawmakers around the world as well as governments, academics and international judges have the right to nominate candidates for the Nobel Peace Prize, and recommendations do not mean that they will be selected.

China put heavy pressure on Norway, whose parliament appoints members of the Nobel Committee, after the prize went in 2010 to jailed dissident writer Liu Xiaobo -- who was still in state custody when he died seven years later.