US lawmaker Chris Smith, who for decades has prodded Beijing to improve its human rights record, announced Tuesday he was creating a congressional Hong Kong caucus to monitor China's actions there.

With thousands of Hong Kong demonstrators clogging the territory's streets and showing no let up in their campaign for free elections, members of the House and Senate have spoken out in support of the protestors and calling on Beijing to refrain from using violence and to respect rights of expression and assembly.

"The freedom genie cannot be stuffed neatly back into the communist bottle," Smith, chairman of the House Foreign Affairs subcommittee on global human rights, said in a statement.

"Beijing can accept this fact, work within the promised One Country, Two Systems Model and be lauded for its leadership, or Beijing can use force and repression again to stifle peaceful dissent and reform, losing the trust of the people of Hong Kong, neighboring countries, and the international community at large."

Smith said he was starting the caucus to monitor human rights in the former British colony, and to ensure that tenets of a 1992 bill calling for Hong Kong to have a "high degree of autonomy" is respected.

"In a world where there are many forces seeking to violently roll-back liberty and human rights, China must choose on which side it will stand," Smith said.

The veteran lawmaker routinely berates Beijing for its record. In 2012 in a dramatic moment, Smith arranged for blind Chinese dissident Chen Guangcheng to phone in to a US congressional hearing from China to address concerns about his safety.

Meanwhile, Senate Foreign Relations Committee chairman Robert Menendez wrote a letter to Hong Kong Chief Executive Leung Chun-ying expressing "grave concerns" about the latest events.

"The recent decision (to prohibit voters from selecting nominees to stand for chief executive) and the suppression and intimidation of peaceful protestors and opposition media -- decisions that have now led to the protests in the streets of Hong Kong -- indicates that Beijing has reneged on the promises it made to the people of Hong Kong," Menendez wrote.