CBS Local — Las Vegas residents are answering the call to help the hundreds of victims in Sunday night’s mass shooting. Crowds of people have been lining up at local hospitals and clinics to donate blood since emergency workers put out pleas for help in the wake of the worst shooting in U.S. history.

Many people have been documenting the massive response to the tragedy in Las Vegas, posting pictures of the people coming out to give blood for the more than 500 wounded at Sunday’s music festival. Over 500 people reportedly stood in line at one United Blood Services center in the city Monday morning.

“There is no substitute for blood,” United Blood Services’ Paul Milakeve said during a 2015 interview. “Everyone relies on volunteer blood donors. It’s a relatively simple and painless process.”

Although waiting on line and passing a standard health screening can take several minutes, the process of donating a pint of blood can take as little as 10 minutes. As good samaritans continue to pour in to help the wounded, many are turning to social media to call on the country to do what they can to help the victims of Las Vegas.

According to the Red Cross, donors must be at least 17 to give blood. Some states do allow 16-year-olds to donate if they have their parent’s permission. All donors are required to weigh more than 110 pounds regardless of age.