There was mucous around Gray's nose and mouth, Porter testified. He and another officer tried to assist Gray. They placed Gray in a "lifesaving position" and waited what "felt like an eternity" for a medic to arrive, Porter told the jury.
He is the first of the six officers to be tried.
With one of his attorneys at times demonstrating Gray's position, Porter testified that he never believed Gray needed immediate medical attention until the sixth and final stop.
"Are you sorry Freddie Gray died?" one of his lawyers asked.
"Absolutely," said Porter, who had testified that he knew Gray from the West Baltimore neighborhood he patrolled.
"Freddie Gray and I weren't friends, but we had a mutual respect," Porter said.
He added, "Any kind of loss of life, I'm sorry to see that."
Earlier, Porter testified that he did not call for a medic before that final stop because Gray had not exhibited signs of a medical emergency.
"I didn't see anything externally, cuts or wounds," Porter said.
Asked whether Gray said he couldn't breathe, Porter replied: "Absolutely not."
Porter's four hours on the stand Wednesday came a day after the prosecution rested a weeklong case Tuesday in the death of Gray, who authorities said broke his neck while being transported in a police van, shackled but not wearing a seat belt.
Porter has pleaded not guilty to charges of involuntary manslaughter, second-degree assault, misconduct in office and reckless endangerment.
'He did nothing'
Prosecutors allege that Porter, summoned by the driver to check on Gray during a stop on the way to a police station, did not immediately call for a medic when Gray asked for help. They also allege that Porter, after seeing that Gray was not buckled in, did not get him into a seat belt as was department policy.
"This defendant did nothing to get him a medic or get him to the hospital. He did nothing when he could have saved a man's life," State's Attorney Michael Schatzow said in court Tuesday.
Gray's April 12 arrest in Baltimore was captured in bystander videos. Prosecutors said Porter, 26, was present for all but one of six stops on Gray's 45-minute ride to the Western Division police station.
Gray's death a week after the injury sparked outrage and demonstrations, some of which were plagued by arson, vandalism and looting despite his family's pleas for peace.
Mayor Stephanie Rawlings-Blake joined Police Commissioner Kevin Davis at a news conference Wednesday to ask the public to respect the judicial process.
"Whether you agree or whether you disagree with the jury's ultimate verdict, our reaction needs to be one of respect in Baltimore's neighborhoods, and the residents and businesses who make up our city should reflect that respect," she said.
On the stand, Porter said the only time he heard a man say he couldn't breathe that day was when Gray was initially arrested. He said he did not know it was Gray at the time.
Porter said he saw the police wagon shaking side-to-side and heard kicking during the second stop.
On the fourth stop, Porter said, the wagon's driver asked him to check on Gray, who was facedown and asking for help. He helped the prisoner sit on a bench, with Gray "supporting his own head," Porter said.
Some prisoners exhibit what Porter called "jail-itis," feigning injury to avoid going straight to the lockup. Gray did not say he was having trouble breathing, Porter repeated.
The officer said he told the wagon's driver, Officer Caesar Goodson, that Gray wanted to go to a hospital. He did not see Gray again until the final stop.
Seat belts not typically used in police wagons
Porter, who joined the Police Department in 2010, testified that he used seat belts when transporting people in his cruiser but never in the tight quarters of the wagon in which Gray sustained the fatal injuries. Part of the reason, he said, was officer safety.
Of the roughly 150 prisoners he put in police wagons during his time on the job, Porter said, none was secured with a seat belt.
"It is the responsibility of the wagon driver to get the prisoner from point A to point B," he told the jury.
During his six weeks of field training, Porter testified, people transported in wagons were never secured with seat belts. At the police academy, he said, cadets were told to use seat belts while transporting prisoners, but were not shown how to do so.
During cross examination, chief deputy state's attorney Schatzow pressed Porter on why an investigating officer noted that Porter told her Gray said he couldn't breathe on the fourth stop. Porter said she misunderstood.
"When she said tell me what happened," Porter testified, "I started at the beginning."
Schatzow asked Porter whether a phrase the officer used -- the "stop snitching culture" -- was the reason why the officer didn't identify the cop who placed Gray in the wagon.
"I'm actually offended you would say something like that," Porter responded. "I gave all of the officers' names."
Schatzow closed his cross examination: "It's ingrained in you to protect life, but at stop four and stop five you did nothing to protect Freddie Gray's life, did you?"
It remains unclear exactly how Gray was injured, but Dr. Carol Allan, the assistant state medical examiner, testified Monday that Gray probably received his neck injury between the van's second and fourth stops.
Although there is conflicting testimony about when, witnesses said Gray complained at least once of being unable to breathe. Gray asked Porter for medical assistance when the officer checked on him at the fourth stop, according to Allan's testimony and Porter's interview with department investigators. Allan said Gray probably was injured when the van stopped suddenly.
The delay in getting Gray medical attention led Allan to classify Gray's death as a homicide.
"If he had gotten prompt medical attention, it would not have been a homicide," she stated, adding that Gray probably would have survived if van driver Caesar Goodson had rushed him straight to a hospital when he told Porter "I can't breathe."
On Wednesday, Dr. Vincent Di Maio, a forensic pathologist testifying for the defense, said Gray's death should have been declared an accident and that the fatal injuries likely occurred during later stops on the police wagon.
"It was just an accident and accidents happen," he said.
Di Maio said he believes Gray suffered a "single catastrophic event" between the fifth and sixth stops -- not between the second and fourth, as the prosecution claims -- that left him paralyzed and unable to speak.
The trial is expected to end by December 17.