Early indications suggested the plainclothes policeman came to the rescue of a French supporter of the Israeli team who was attacked by PSG fans outside the stadium.
A group of up to 150 fans then turned on the policeman.
At first he let off a tear gas canister to try to disperse his assailants, then fired his gun, killing one man immediately and injuring another.
Sources said the PSG fans appeared to be far-right sympathisers and were chanting "France for the French" as they confronted the Hapoel supporter. The lone policeman was black.
The Paris prosecutor's office was expected to open a formal investigation into the killing on Friday.
The explosion of violence came after PSG suffered a shock 4-2 loss to their Tel Aviv opponents.
