Leclerc attempted to squeeze up the inside of Vettel on the approach to turn three, only to strike a kerb, sending his car onto the rear wing of Vettel's SF1000.
Vettel limped back to the pits to retire, while Leclerc managed to continue after a front-wing change, only to be forced out himself soon as there was too much damage to his car.
After both drivers spoke in the television pen where interviews are conducted, Leclerc took the blame.
"I apologised," said Leclerc. "Excuses are not enough in times like this. I am just disappointed in myself. I've done a very bad job today. I let the team down. I can only be sorry even though it's not enough
"I hope I will learn from this and I will come back stronger for the next races. It's a tough time for the team, we don't need that, the team doesn't need that, and I put all the efforts of the team in the bin."
Vettel said he was "very surprised" to find Leclerc on his inside at such an early stage of the race, and with the midfield bunching into such a tight corner.
"I was fighting two other cars, we were already three cars into turn three," said Vettel.
"I was very surprised because I had the inside and I wasn't expecting Charles to try something. I didn't think there was any space.
"Obviously, a big pity. It's something we should avoid, but there was not much I could have done differently.
"I was already taking it easy and conservative because it's already very busy, a hairpin, it's very tight, I was trying to place my car for the next straight but by then I realised I had quite some damage on the car/."
Asked whether he felt Leclerc was too optimistic, Vettel said: "I don't think there was space so that's why we collided."
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