A lawmaker on Monday urged the United States to intervene to protect a popular activist site after it was hit by China-based hackers apparently upset at a petition to free artist Ai Weiwei.

Change.org, an online activist network, has been disrupted intermittently by cyber-attacks over the past week after the heads of major museums posted a petition for China to free Ai, an outspoken critic who was seized April 3.

In a letter to Secretary of State Hillary Clinton, Representative Rosa DeLauro said it was vital to ensure Internet freedom as millions of Americans relied on Change.org and other sites for political participation.

"I believe this attack on Change.org from outside the United States is an attack on Americans' fundamental right to free speech and another example of the government of China's intent to restrain human rights," she said.

DeLauro, a member of Clinton's Democratic Party from Connecticut, urged the State Department to condemn the incident and "ensure that this attack and others like it in the future are stopped swiftly and that the perpetrators are brought to justice."

More than 120,000 people have signed the Change.org petition for China to free Ai, a world-acclaimed artist known for his "Sunflower Seeds" exhibition at London's Tate Modern of millions of subtly unique mini-sculptures.

Chinese authorities had begrudgingly tolerated Ai's social commentary. But he was taken into custody at Beijing's airport as China launches a major clampdown on dissent amid the democracy uprisings in the Middle East.

Clinton has repeatedly urged Internet freedom around the world, saying in a speech in February that countries risk a public backlash of the type seen in Egypt and Tunisia if they suppress online activity.

But a recent Senate report urged a more robust US response to Internet freedom, warning that China's leaders were becoming increasingly sophisticated at silencing dissenting voices both at home and abroad.

New York's Guggenheim Museum launched the online petition for Ai which was signed by leaders of major art institutions around the world.