Spokeswoman Brenda Jones told CNN that Lewis is under routine observation but she did not give details of the nature of his illness or where he is hospitalized.
CNN affiliate WSB-TV reported that Lewis was being treated at a hospital in metro Atlanta, and the station quoted unnamed sources as saying Lewis became ill on a flight to Atlanta on Saturday.
Lewis, 78, has represented Georgia’s 5th Congressional District, which includes much of the city of Atlanta, since first being elected in 1986.
But as the nonviolent demonstrators approached the Edmund Pettus Bridge, on their way to the state capital in Montgomery, they were confronted by heavily armed state troopers and local police. When the march refused to turn back, protesters were set upon and beaten.
The images from “Bloody Sunday” — and Lewis, who was photographed under assault and suffered a fractured skull — helped galvanize support for the Voting Rights Act, which was signed into law by President Lyndon B. Johnson months later.