In his May Day message carried by state media on Thursday, Senior General Than Shwe, the country's leader, urged workers to back the new constitution.
But Bush said "Than Shwe and his regime are ensuring that the referendum vote will be on a dangerously flawed constitution, and will not be free, fair, or credible".
"They continue to ignore calls from the Burmese people and the international community."
Forced to vote
The Reuters news agency on Friday quoted witnesses saying that hundreds of government workers have already been forced to vote in favour of the new constitution in open ballots, more than a week before the May 10 referendum.
In one case, about 700 employees in the electric power ministry's Yangon office were forced to tick their ballot papers with local referendum officials observing, witnesses said.
"We were all shocked and some people were furious but they couldn't do anything," said one witness. "They said those who wanted to vote 'no' had to hand in their resignation."
State firms targeted
On Thursday, Bush signed an order to freeze the assets of state-owned firms in Myanmar, formerly known as Burma.
"These companies, in industries such as gems and timber, exploit the labour of the downtrodden Burmese people, but enrich only the generals," Bush said on Thursday.
"Over the past eight months, my administration has tightened sanctions on the regime.
"We've imposed visa bans on the junta's generals and their families and their cronies, trying to send a clear message, and we hope the rest of the world follows as well."
The new US law allows the Bush administration to crack down on enterprises owned by the government of Myanmar.
Previously, the Bush administration was only authorised to target individuals and privately-owned companies in Myanmar.