“I have not asked for – and do not need – federal intervention,” Portland’s mayor Keith Wilson said earlier this month.
The Immigration and Customs Enforcement (ICE) facility in Portland has been targeted by protesters, sometimes leading to violent clashes in which some federal agents have been injured.
Trump’s Saturday morning post does not specify whether he intends to activate national guard or regular US military.
The BBC has contacted the Department of War for further details.
Trump hinted at an operation in Portland on Thursday.
“We’re going to get out there and we’re going to do a pretty big number on those people in Portland,” he told reporters in the Oval Office, describing the targets as “professional agitators and anarchists”.
He described Portland earlier in September as “like living in hell”, and had said that he was considering sending in federal troops.
Earlier this week, Trump also signed an order formally designating antifa as a domestic terrorist organisation.
Democratic Oregon lawmakers have denounced Trump’s comments, as well as alleged conduct by ICE agents in the state.
“ICE has said they’re targeting people for arrest and detainment who have committed crimes. That’s what they told us. But that’s not what we are seeing,” said Democratic house representative Suzanne Bonamici to reporters on Friday.