Cristiano Ronaldo said he will "never change" following his angry outburst after the Portugal captain was denied a dramatic late winner in contentious circumstances against Serbia.

Ronaldo was convinced his last-gasp effort crossed the line, but the officials gave the all clear as 10-man Serbia rallied from two goals down to secure a 2-2 draw in World Cup qualifying on Saturday.

Stefan Mitrovic made back-to-back goal-line clearances to thwart Portugal in stoppage time, though Ronaldo was left fuming as the superstar was adamant his effort crossed line in Belgrade, where there was no VAR in place to review the decision.

Ronaldo was booked for his protestations before he angrily threw his captain's armband on the floor and walked off the field with just seconds remaining.

Afterwards, Ronaldo used Instagram and social media to discuss the controversial incident away to Serbia in the Group A showdown.

"Being captain of the Portugal team is one of the greatest pride and privileges of my life," Ronaldo wrote. "I always give and will give everything for my country, that will never change.

"But there are difficult times to deal with, especially when we feel that an entire nation is being punished. Lift your head and face the next challenge now! Come on, Portugal!"

Portugal head coach Fernando Santos revealed "embarrassed" referee Danny Makkelie apologised post-match in the Serbian capital.

"The referee apologised to me in the booth and told me he was embarrassed," Santos said during his news conference. "I was in the dressing room with him and he apologised to me. He had told me on the pitch that he was going to see the images and that if it was the case he was calling me to apologise ... and so it was.

"It is the second time in a qualifying phase that they apologise after the game ... I told him on the field that such a test there is no VAR or goal line ... Incidentally, one of the things he told me was that the goal line was important… but I told him that the ball was half a metre inside the goal. There was no obstacle between the goalkeeper and the goal line.

"We have to rethink. The referees are human and make mistakes, but that's why there is the VAR and the goal-line technology to avoid this."

The result left Serbia and Portugal level on four points atop Group A, one point ahead of Luxembourg through two matchdays.

On the incident, Serbia boss Dragan Stojkovic added: "I never comment on referee's decisions whether they go our way or not. Mistakes by officials are part of the game and should be viewed as such. I didn't see it clearly, I can only pass a verdict after I've seen the replay.

"Ronaldo is a great player but we largely contained him tonight and in the second half we showed true grit and character to fight back from two goals down.

"We always knew what to expect from a top team like Portugal but against such opposition you have to be brave and never give up. This group of players is quickly learning that."