Unsigned Colin Kaepernick chose not to attend an NFL-arranged private workout, instead holding his own session at a high school to allow media to attend.
He said he wanted "transparency", claiming the NFL denied media access to its own planned workout in Atlanta.
All 32 clubs were invited by the NFL to assess Kaepernick, who last played in 2016, when he began kneeling at the US national anthem in protest at racism.
The NFL said it was "disappointed" the venue changed at 30 minutes' notice.
Representatives from 25 clubs had turned up at the Atlanta Falcons' training facility expecting to see the 32-year-old.
"Kaepernick was a no-show for 25 NFL teams at Falcons facility waiting at 3pm," the league tweeted.
Addressing a media huddle when his workout finished at Charles R Drew High School, in Riverdale, Kaepernick said: "Our biggest thing was making sure we had transparency in what went on.
"We weren't getting that elsewhere, so we came out here. It's important you're all here [the media]. You've all been attacked for the last three years, you continue to be attacked. We appreciate what you all do."
The former San Francisco 49ers quarterback added: "I've been ready for three years - I've been denied for three years.
"We have nothing to hide. So we're waiting for the 32 owners, the 32 teams, [NFL commissioner] Roger Goodell, to stop running. Stop running from the truth, stop running from the people.
"My agent is ready to talk to any team; I'll continue to be ready.
"We'll let you know if we hear from them. The ball is in their court. We're ready to go."
Kaepernick's choice of venue on Saturday was an hour from the scheduled location and the NFL said on Twitter: "No-one got a heads up until NFL saw the [Kaepernick] Twitter statement."
In a long statement, the league said it had made considerable effort to work with Kaepernick's representatives, adding: "We are disappointed that Colin did not appear for his workout.
"Colin's decision has no effect on his status in the league. He remains an unrestricted free agent eligible to sign with any club."
On announcing the venue change, the statement from Kaepernick's representatives said: "From the outset, Mr Kaepernick requested a legitimate process and from the outset the NFL league office has not provided one.
"Most recently, the NFL has demanded that as a precondition to the workout, Mr Kaepernick sign an unusual liability waiver that addresses employment-related issues and rejected the standard liability waiver from physical injury proposed by Mr Kaepernick's representatives."
Kaepernick, who became a free agent after the 2016 season and remains unsigned, had also requested all media be allowed into the workout to observe and film it, but the NFL denied the request, their statement added.
He filed a grievance against NFL owners in October 2017 after going unsigned as a free agent, believing they were conspiring not to hire him because of his kneeling protests.
The two sides resolved the grievance in February under a confidentiality agreement.