A Florida man who mailed pipes filled with explosives to prominent Democrats, as well as other opponents and critics of United States President Donald Trump, has been sentenced to 20 years in prison.

Key points: Investigators found that Sayoc's devices had no mechanism to trigger an explosion

Investigators found that Sayoc's devices had no mechanism to trigger an explosion Defence lawyers noted Sayoc's learning disabilities, childhood abuse and isolation

Defence lawyers noted Sayoc's learning disabilities, childhood abuse and isolation The judge believed Sayoc did not intend to kill, but said the crimes were "far too horrible"

Cesar Sayoc, 57, who pleaded guilty in March to using weapons of mass destruction and other crimes, began crying in the US District Court in Manhattan when Judge Jed Rakoff read out the sentence.

Prosecutors had asked Judge Rakoff to sentence Sayoc to life in prison, while the defendant's lawyers had sought a sentence of 10 years plus one month.

Ian Marcus Amelkin, one of Sayoc's lawyers, argued that Sayoc's crimes were caused by paranoid, delusional thinking brought on by heavy use of steroids.

Sorry, this video has expired Michael Moore released footage of Cesar Sayoc at a Trump 2020 rally in Melbourne, Florida in February 2017.

He said Sayoc, who was living in a van at the time of his arrest last year, had struggled all his life with severe learning disabilities, the effects of childhood abuse and social isolation.

Sayoc, Mr Amelkin said, saw Mr Trump as a "father figure" and became obsessed with conspiracy theories about people he perceived as being enemies of the president.

"We believe that the President's rhetoric contributed to Mr Sayoc's behaviour," he said.

Assistant US Attorney Jane Kim, one of the prosecutors, argued that Sayoc was a danger to the public and that his crimes warranted severe punishment.

"The defendant here set out to terrorise people," she said.

Sayoc's lawyers said that he saw President Trump as a father figure. ( Reuters: Geo Rodriguez )

Sayoc expressed remorse shortly before the sentencing.

"I am so very sorry for what I did," he said.

Sayoc sent packages containing pipes stuffed with explosives, wires and alarm clocks to 16 intended targets, including former president Barack Obama, former secretary of state Hillary Clinton, US senators Cory Booker and Kamala Harris, billionaire investor and Democratic donor George Soros, former Central Intelligence Agency director John Brennan, actor Robert De Niro and CNN.

Sorry, this video has expired CNN anchors were interrupted by an on-air bomb scare.

All of the devices were intercepted and none exploded.

A report by federal investigators concluded they had no mechanism to trigger an explosion and "would not have functioned as designed."

Prosecutors argued that Sayoc nonetheless meant to injure his targets, while his lawyers said he meant only to inspire fear.

Sayoc himself said in a handwritten letter to Mr Rakoff after his guilty plea that the bombs were a "hoax."

Mr Rakoff said he believed Sayoc.

"He hated his victims. He wished them no good, but he was not so lost as to wish them dead, at least not by his own hand," the judge said.

Still, the judge said, Sayoc's crimes were "far too horrible" for a "relatively lenient" 10-year sentence.

Mugshots of Cesar Sayoc from 2004, 2004, 2013 and 2015. ( Reuters )

Reuters