Hillary Clinton: Our grief isn't enough, we must stand up to the NRA

Hillary ClintonHillary Diane Rodham ClintonWhite House protests extend into sixth day despite rain Clinton: US is 'losing friends and allies' under Trump Justice Dept releases surveillance applications for former Trump aide MORE on Monday pushed for action after a gunman opened fire at a music festival in Las Vegas, killing at least 59 people and injuring hundreds more.

"Las Vegas, we are grieving with you—the victims, those who lost loved ones, the responders, & all affected by this cold-blooded massacre," Clinton tweeted.

But Clinton said it's not enough to grieve and urged people to stand up to the National Rifle Association (NRA).

"The crowd fled at the sound of gunshots. Imagine the deaths if the shooter had a silencer, which the NRA wants to make easier to get," she tweeted.

"Our grief isn't enough. We can and must put politics aside, stand up to the NRA, and work together to try to stop this from happening again."

At least 50 people were killed and 400 more injured when a gunman opened fire Sunday night at a country music festival in Las Vegas.

The suspected gunman was identified as Stephen Paddock, 64, a Nevada resident.

Several lawmakers have since called for Congress to take action on gun violence in the wake of the deadliest mass shooting in  U.S. history.

President Trump earlier Monday expressed his sympathies to the victims of the attack.