By Susan Kelly

CHICAGO (Reuters) - The worst mass shooting in modern U.S. history has prompted the American Medical Association to call gun violence a "public health crisis" and urge that Congress fund research into the problem.

The AMA, which lobbies on behalf of doctors, said on Tuesday it will press Congress to overturn 20-year-old legislation that blocks the Centers for Disease Control and Prevention from conducting research on gun violence.

A 29-year-old gunman slaughtered 49 people at a gay nightclub in Orlando, Florida, before dawn on Sunday.

The AMA adopted the policy at its annual meeting in Chicago. It called U.S. gun violence a crisis that requires a comprehensive response and solution.

"With approximately 30,000 men, women and children dying each year at the barrel of a gun in elementary schools, movie theaters, workplaces, houses of worship and on live television, the United States faces a public health crisis of gun violence," Dr. Steven Stack, AMA president, said in a statement.

"Even as America faces a crisis unrivaled in any other developed country, the Congress prohibits the CDC from conducting the very research that would help us understand the problems associated with gun violence and determine how to reduce the high rate of firearm-related deaths and injuries."

Congress placed restrictions on CDC funding of gun research into the federal budget in 1996 at the urging of gun rights supporters who claimed the agency was biased toward gun control.

AMA has several long-standing gun safety policies including support of legislation that calls for a waiting period before the purchase of any form of firearm in the United States. It also supports background checks for all handgun buyers.

(Reporting by Susan Kelly in Chicago; Editing by Caroline Humer and Matthew Lewis)