As the first hurricane-force winds and sheets of rain slammed onto the island on Monday, Castro travelled to the western province of Pinar de Rio, where he personally led emergency operations.
Measures include the biggest evacuation in the country's history, in which 1.6 million people moved to safer ground, some even staying in air raid shelters.
Cuba already suffered an estimated one billion dollars in damages from Hurricane Charley last month, but Castro has said he would not accept any aid from the US or any other country "that has adopted sanctions against Cuba."
Ivan on Monday packed sustained winds of 260km an hour, with higher gusts.
Its projected track took it near Mexico's Yucatan peninsula, and later this week towards the United States, where forecasters said it could make landfall in Florida, Alabama, Mississippi or Louisiana.
The power of the storm, a rare top-scale category five hurricane, could change in coming days, but in any case, "Ivan is forecast to be a major hurricane at US landfall," said Miles Stewart, a forecaster at the Miami-based National Hurricane Center.
If it slams onto the US coast at its present strength it would be only the fourth category five hurricane to hit the United States since 1935.
In Florida, Governor Jeb Bush has extended a state of emergency over the entire state, which is still mopping up from the ravages wrought earlier this month by Hurricane Frances and last month by Charley.
Residents in northwestern Florida, the most likely area for Ivan's US landfall, were busy boarding up their homes and businesses as authorities opened emergency shelters.