(Updates with latest outages data, adds details)

Aug 30 (Reuters) - Entergy Corp said crews have begun assessing damage where it is safe to do so now that Hurricane Ida, which weakened to a tropical depression on Monday, has passed through the area, as major utilities in Louisiana grapple with restoring power outages.

Road closures, flooding and other accessibility challenges due to the storm could delay restoration in some areas, Entergy Louisiana LLC posted on its website, adding that customers in the hardest-hit areas could experience power outages for weeks.

Based on historical restoration times, customers in the direct path of a storm as intense as Hurricane Ida could experience outages for more than three weeks, it added.

"While 90% of customers will be restored sooner, customers in the hardest-hit areas should plan for the possibility of experiencing extended power outages."

Nearly all of Louisiana lost electrical power on Monday after one of the most powerful hurricanes to strike the region downed power lines, littered roads with debris and flooded isolated communities south of New Orleans.

Entergy Louisiana said there were about 814,155 customers affected around 5 p.m. on Monday, while major transmission lines that deliver power into the New Orleans area are currently out of service.

"At 8 a.m. across our service area, 216 substations, 207 transmission lines, and more than 2,000 miles of transmission lines are out of service. We know of one transmission line that spans the Mississippi River that is down."

Around 29,643 Entergy customers were without power in Mississippi.

Meanwhile, there were some 96,938 Cleco Power customers without power by Monday evening. Cleco Power serves around 290,000 customers in 24 of Louisiana's parishes and supplies wholesale power in Louisiana and Mississippi, according to the company website.

"Crews were working earlier today, but all restoration work in south Louisiana has stopped due to heavy rain and high winds," Cleco said. "As soon as the storm passes and conditions are safe, we will begin assessing damage and resume our restoration work."

(Reporting by Swati Verma in Bengaluru; Editing by Christopher Cushing Editing by Matthew Lewis)