Trump tweets video showing violent protests

US presidential aspirant and Republican front-runner Donald Trump tweeted a video that features clips of anti-Trump protests, violence and a bloodied police officer before a dramatic narration, "We're at war".

The video posted by the business mogul on Monday splices a clip of a gun firing into the air with footage from a cancelled Trump rally in Chicago, during which fist-fights broke out between Trump supporters and protestors, CNN reported.

There has never been a report of a gun being fired in connection with a Trump rally or protests.

"We're at war. No one wants to admit it but humanity is under attack," the narrator, whose voice can be recognised as that of actor Martin Sheen, says over rolling clips of protests.

"One very specific man might be all that stands between humanity and the greatest threat of our brief existence."

The narration is pulled from Sheen's voice-overs for the video game Mass Effect 2.

The video was originally posted last week on the website Vidme by an account named "trumpeffect".

Trump's social media director Dan Scavino also shared the video on Twitter late Monday night.

The video includes chaotic scenes and a clip of Democratic presidential front-runner Hillary Clinton laughing, President Barack Obama lifting weights and images of the Moon landing.