A Melbourne man has described the harrowing moment he confronted a passenger on board a Malaysia Airlines flight who threatened to "blow the f***ing plane up".

Former AFL player Andrew Leoncelli told the ABC he was sitting four rows from the front of the plane when a man ran up and tried to enter the cockpit shortly after take-off.

"He started saying, 'I need to see the pilot. I need to see the pilot'," Mr Leoncelli said.

"The staff were saying, 'Sit down. Go back to your seat. Go back to your seat.'

"Then he started going louder: 'No I need to see the pilot.' And got louder and louder and eventually they screamed for help.

"So that's when I jumped up."

Sorry, this video has expired Police inside the cabin of the Malaysia Airlines flight

Mr Leoncelli said he confronted the man, who was holding a large object with what appeared to be antennas coming off it.

Victoria Police later said the incident was not being treated as terrorism and that the man's object was not a bomb.

"[He was] saying, 'I'm going to blow the plane up, I'm going to f***ing blow the plane up'," Mr Leoncelli said.

"I said, 'Mate, get back to your f***ing seat.'

"I was literally eyeballing him and he was like, 'I'm going to blow the f***ing plane up, I'm going to blow the plane up'.

"He was agitated, is the best description.

"He was a man on a mission and he was pacing from one foot to the other, like a cat on a hot tin roof."

The passenger was restrained by the time police entered the plane. ( Supplied: Andrew Leoncelli )

Mr Leoncelli said the man then ran towards the back of the plane and airline staff gave chase.

"What we've learnt is that two or three brave young Aussies have taken him on and got him to the ground, pulled the thing out of his hand."

"[They] basically jammed his head into the carpet and put the restraints things on him and then sat on him.

"Staff grabbed the object, which we're not sure what it was, he was claiming to blow the plane up with, and walked it back to the front of the plane."

Mr Leoncelli said the object looked large, black and metallic and was about the size of a watermelon.

"[There were] two sort of short, antenna things and it looked like it had a socket for an iPhone to go into," he said.

"I don't know whether that was a speaker system or not but there's no chance he got this on the plane without taking it through security, because this was the size of a giant bowling ball."

The plane made an emergency landing at Melbourne Airport and Mr Leoncelli said all passengers were taken into a holding area and told they would all be interviewed.