By Robbie and Ward

TUPELO, Mississippi (Reuters) - A storm system that killed more than 20 people continued its march across a large swath of the U.S. Southeast on Tuesday, packing baseball-sized hail, damaging winds and the threat of more tornados, meteorologists said.

Reports of the death toll varied, but CNN has put it at 29 lives across six states, and scores of people have been injured.

The threat of tornadoes could last for several days as the strong weather system with a large area of unstable air lashed the southern and central United States.

Early on Tuesday morning, a large cluster of thunderstorms pelted Alabama, Georgia and the Florida Panhandle, with tornados likely touching down along the Alabama-Georgia state line. The tornado threat in those areas will briefly weaken before rising again in Tennessee, Mississippi and Alabama later on Tuesday morning and afternoon.

"The main severe threat today is down in the central and eastern Gulf Coast," National Weather Service meteorologist Brynn Kerr said, adding that there was potential for severe conditions in Tennessee and the Ohio Valley later on Tuesday.

Georgia Governor Nathan Deal, who declared a state of emergency late on Monday evening in preparation for the looming storms, said in a statement: "We're prepared now, and we'll be ready for recovery should we, God forbid, experience tornado damage or flooding."

Most of the deaths from the severe storm system occurred on Sunday, when tornadoes tossed cars like toys in Arkansas and other states.

One of several twisters to tear across Mississippi struck Tupelo on Monday, damaging hundreds of homes and businesses, downing power lines and shredding trees, the National Weather Service said. At least one person was killed in Tupelo, where residents grappled with widespread power outages and an 8 p.m. (0100 GMT Tuesday) curfew.

Tupelo Police Chief Bart Aguirre said emergency crews were going house to house, searching damaged buildings, and some residential areas were closed off as emergency crews checked downed power lines and gas leaks. Some residents whose homes were destroyed took refuge in a Red Cross shelter at a downtown sports arena.

Officials were also picking through the rubble in Lincoln County, Tennessee, near the Alabama state line state late Monday evening, with reports of a tornado that had likely damaged Lincoln County High school and area homes, National Weather Service Meteorologist Mark Richards said.

There also appeared to be a tornado that hit about 80 miles east of Nashville.

The storm system later pushed into parts of Alabama, where emergency officials said at least two people were killed at a trailer park near Athens.

At least 15 people had died in Arkansas in the storm that authorities said produced the first fatalities of this year's tornado season, Governor Mike Beebe said.

The White House said President Barack Obama, who has been on a trip abroad, called Beebe to receive an update on the damage and to offer condolences. Medical officials reported at least 100 people in Arkansas were injured.

One person was killed in neighboring Oklahoma and another in Iowa, authorities said.

(Reporting by Robbie Ward in Tupelo, Mississippi; Emily LeCoz in Oxford, Mississippi; Steve Barnes and Suzi Parker in Little Rock, Arkansas; Verna Gates in Birmingham; Kevin Gray in Miami; and Tim Ghianni in Nashville, Tennessee; Writing by Eric M. Johnson; Editing by Lisa Von Ahn)