By Lisa Maria Garza and Jim Forsyth

DALLAS (Reuters) - Texas Governor Greg Abbott sought a presidential disaster declaration on Friday after severe weather this week killed at least 21 people, caused massive flooding and prompted evacuations.

Storms that battered North Texas on Thursday and Friday added more runoff to swollen rivers and prompted hundreds of calls for help in Dallas, where some areas saw up to seven inches (17.8 cm) of rain.

"Communities across the State of Texas have experienced devastating destruction, injury and – most tragically – loss of life due to the major and (an) unceasing severe weather system that has been impacting our state for weeks," said Abbott, who has declared 70 counties disaster areas.

The presidential declaration would free up federal funds to help rebuild. No estimate has been given for the damage in Texas, which has a $1.4 trillion a year economy and is the biggest domestic energy provider.

Near Dallas on Friday, thousands of cars were trapped for about six hours on a suburban freeway blocked by floodwaters. The Red Cross distributed Girl Scout cookies and water to stranded motorists.

"I feel like I am on an island and nobody cares," Vanessa Paterson, who was on the highway with her 6-month-old son, told TV station WFAA.

The National Weather Service issued a flash flood watch from Central Texas into Missouri, with the additional rain tipping off a new round of flooding.

Dallas officials advised people to go home early and stay off streets that have seen more water than they can handle.

"This would be a great night to stay home, watch a movie and cook some popcorn," Dallas Police Deputy Chief Scott Walton told a news conference.

The mayor of Wharton, about 60 miles (100 km) southwest of Houston, issued a mandatory evacuation order for about 900 people living near the Colorado River, which began flooding into the city on Thursday and has been rising steadily since.

Hundreds of Texans are set to spend the night in shelters after this week's floods turned streets into rivers, ripped homes off their foundations and swept over thousands of vehicles.

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The rushing water trapped people in cars and houses. One Dallas-area police officer had to be plucked to safety by a helicopter.

The rainfall for May across the state has already set a new record and more storms were forecast for Texas over the weekend.

The Brazos River, which began overflowing its banks on Wednesday in Parker County about 30 miles (50 km) west of Fort Worth, is expected to see another surge due to recent rains.

"This situation will get worse before it gets better," said Parker County Emergency Management Coordinator George Teague.

(Writing by Jon Herskovitz in Austin, Texas; Reporting by Lisa Maria Garza in Dallas and Jim Forsyth in San Antonio; Additional reporting by Marice Richter in Dallas and Heide Brandes in Oklahoma City; Editing by Lisa Lambert, Doina Chiacu and Sandra Maler)