Houston Mayor Bill White has called for residents to leave low-lying areas of the city as Hurricane Rita approaches.
Texas residents are bracing themselves for the worst
Mr White has warned there are not enough government vehicles to evacuate everyone in the affected areas, and urged friends and neighbours to help.
Rita is now a Category Five storm, with sustained winds of 165mph (265km/h), making it stronger than Hurricane Katrina which devastated New Orleans.
New Orleans is again on high alert, as the storm could cause yet more floods.
Hurricane Rita, which is moving across the Gulf of Mexico after battering the coastline of Cuba and the southern tip of Florida, was upgraded twice on Wednesday. Category Five is the highest level on the scale.
The storm is expected to hit Texas on Saturday, and tens of thousands of people are evacuating coastal towns there.
Troops and supplies are already being brought to the region to deal with the hurricane's aftermath.
US President George W Bush has called on people to heed evacuation orders, especially in New Orleans, where some residents had been allowed to return home in recent weeks.
"Mandatory evacuations have been ordered for New Orleans and Galveston. I urge the citizens to listen carefully to the instructions provided by state and local authorities. And follow them," Mr Bush said.
"We hope and pray that Hurricane Rita will not be a devastating storm. But we've got to be ready for the worst," he added.
Evacuees moved again
The BBC's Alastair Leithead in Miami says state and federal authorities are planning for something as big as Hurricane Katrina, which devastated parts of the Gulf coast last month, and this time they want to be sure they are properly prepared.
Oil companies have stepped up their evacuation of rigs in the region. Galveston, another Texas city, is being evacuated.
Several thousand Louisiana residents who found shelter in Texas after their homes were wrecked by Hurricane Katrina are being uprooted again and moved to Arkansas and Tennessee.
And 500 buses are standing by, ready to evacuate people from New Orleans where protective levees are in a weakened state and could be breached once more by heavy rain.
"The conditions over the central Gulf are much like they were for Katrina," US National Hurricane Center deputy director Ed Rappaport told CNN.
The eye of the storm has yet to make landfall
Earlier, areas of northern Cuba and southern Florida were flooded, power was cut off and winds damaged buildings and infrastructure.
But forecasters said Rita had taken the best possible course, with the eye of the storm missing land altogether.
Florida Governor Jeb Bush had declared a state of emergency in the state, which allows the state to oversee evacuations and call in the National Guard.
In Cuba, some 58,000 people were evacuated from the northern coast and more than 6,000 in Havana alone, Cuban officials said.
The hurricane season runs from 1 June to 30 November.