"Everyone is being asked to stay at home. All schools will be closed. Only workers in the essential services and persons travelling will be allowed free movement," Ramkalwan's office said in a statement.
The explosion had caused "massive damage" to the Providence Industrial zone and surrounding areas on the main island of Mahe, the statement said, while flooding due to rains had resulted in "major destruction".
The statement gave no further details.
The Seychelles is made up of 115 islands and is the least populous country in Africa with about 100,000 people.
(Reporting by Aaron Ross; Editing by Tom Hogue)