MIAMI - Hundreds lined up to give blood Sunday in Orlando to help the victims of the massacre at a gay nightclub, but major restrictions remain for gay men wanting to give blood themselves.

The response overwhelmed OneBlood donation centers, where officials asked donors to make appointments and continue donating over the next several days. Over 50 people were injured and 50 were killed when a gunman opened fire early Sunday inside the downtown Orlando club Pulse.

While many Facebook and Twitter posts from individuals and at least one gay advocacy group in Florida said no one would be turned away and all blood would be screened, OneBlood denied any change in policy.

"All FDA guidelines remain in effect for blood donation. There are false reports circulating that FDA rules were being lifted. Not true," OneBlood tweeted.

In December, the Food and Drug Administration lifted a three-decade-old ban on blood donations from gay and bisexual men. But the lifetime ban was replaced with a new policy barring donations from men who have had sex with a man in the previous year.

The new policy brought the U.S. in line with Australia, Japan, the U.K. and other countries, and researchers said it could slightly increase the U.S. blood supply. Gay rights activists said it still perpetuated negative stereotypes dating to the beginning of the AIDS crisis.

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Messages left Sunday for OneBlood and FDA officials were not immediately returned.

In a tweet, Pulse staff encouraged donations of water, juice and snacks for people waiting in long lines to donateblood in Orlando.

According to the American Red Cross, roughly 38 percent of the U.S. population is eligible to donate blood at any given time, but less than 10 percent of those people actually do so each year.

All U.S. blood donations are screened for HIV, the virus that causes AIDS. The Zika virus has been a more immediate concern, and the FDA asks people to put off donating blood if they have been to outbreak areas, show symptoms of a Zika infection or have had sexual contact with someone exposed to that virus.

In Florida, the risk of a potential Zika virus outbreak has prompted some blood banks to seek more donations to stock up on blood supplies before anyone in the state contracts the Zika virus from local mosquitoes.

OneBlood joined other blood banks in shipping blood products from the continental U.S. to Puerto Rico because of a Zika virus outbreak on the Caribbean island that disrupted blood collections there.