Federal officers who are protecting Portland's courthouse under the orders of the Trump administration have revealed that they 'fear for their lives' after 61 nights of protests in the city, but demonstrators say they are fighting back to protect themselves from the agents.

President Donald Trump sent federal agents to Portland earlier this month and has since said he would be sending 100 more troops into the city to protect federal property.

Demonstrators have taken to the streets to demand justice for black men and women who have died at the hands of police officers across the US, including George Floyd who took his last breath under the knee of a white Minneapolis police officer on May 25.

'It's scary. You open those doors out, when the crowd is shaking the fence, and... on the other side of that fence are people that want to kill you because of the job we chose to do and what we represent,' said a Deputy US Marshal who has been protecting the Mark O. Hatfield Federal Courthouse for weeks.

He requested anonymity because protesters have identified him and posted his personal information online.

'I can't walk outside without being in fear for my life,' he said. 'I am worried for my life, every time I walk outside of the building.'

'I finally get outside at 7am, after being in the building since 3pm the day prior, and I look east and I'm like, "Oh, the world's normal over there and people are driving to work and the city is clean and functioning,"' the Deputy US Marshal said.

Federal property has been increasingly targeted as the city's protests against racial injustice march on. But Trump's decision has added fuel to the fire and re-energized Portland protesters who arrived by the thousands outside the courthouse Monday night.

Within the past few days, federal agents have seen as many as 4,000 people on a single night, the largest total so far.

The US Marshals Service decided last week to deploy 100 deputy marshals to fortify security outside the federal courthouse, according to an internal email obtained by The Washington Post on Monday.

The email indicated that the marshals began arriving last Thursday, ahead of a heated weekend that saw heavily armed federal troops repeatedly fire tear gas, flash bangs and pepper balls into crowds of protesters as they attempted to breach a fence surrounding the courthouse on Saturday and Sunday night.

Trump decried the protests and subsequent media coverage in a pair of tweets on Monday night, writing: 'The Fake News Media is trying to portray the Portland and Seattle "protesters" as wonderful, sweet and innocent people just out for a little stroll.