Almost 500 intellectuals in Vietman had by Wednesday signed an online petition calling for an end to the one-party system and for free elections.
The petition, which outlines an alternative constitution, was published on several popular blogs Tuesday in response to a public consultation launched by the Communist Party this month on proposed amendments to the Constitution.
"The 1992 Constitution was issued in the early years of renewal policy, so it is not suitable for the current situation in Vietnam," former justice minister Nguyen Dinh Loc, one of several former senior officials to have signed the petition, said Wednesday.
"The [petition's] proposed constitution is much better for Vietnam than the current one," he said. "It is better to have a multi-party system now."
"This is a bombshell for the Vietnamese political system," said Nguyen Thu Nga, an office clerk in Hanoi. "The communists should accept this one instead of taking action against intellectuals."
The group's proposed constitution calls for the name of the country to be changed to the Democratic Republic of Vietnam from the Socialist Republic of Vietnam.
It also calls for a separation of powers between the executive, legislative and judicial branches of government.
Several host websites have been blocked to prevent people accessing the proposed alternative constitution.
The authorities have said they will take comments from the public until March 31 on the proposed official revisions to the constitution, which could be adopted by the National Assembly in May.
Sensitive topics such as introducing multi-party elections or improving land rights have been explicitly excluded from the consultation process.