Lille's 5-0 thrashing of relegated Arles Avignon on Saturday had put pressure on champions Marseille to perform and they capitulated just when victory was looking assured.
With five matches left, Lille lead Marseille by a point and have a seven-point advantage over third-placed Lyon who meekly went down 2-0 at Toulouse and had two players sent off.
Mauro Cetto gave mid-table Toulouse a first-half lead before Lyon left-back Aly Cissokho suffered a double calamity, scoring an own-goal in the 67th minute when trying to give the ball back to the goalkeeper and then receiving his marching orders.
He was dismissed for a dangerous tackle five minutes from time with once mighty Lyon already a man down after Brazil's Michel Bastos was booked twice in a minute for fouls midway through the second period.
In Marseille, forward Mathieu Valbuena calmly netted at the far post on 56 minutes as the hosts finally made their dominance pay but South Korea striker Jung latched on to a long ball over the top to score 13 minutes from the end.
"We are disappointed because they hadn't created a single chance all match and they scored with their first," Valbuena said before looking ahead to next Sunday's clash at Lyon.
"It will be tricky because they are in difficulty too. We've got to get back to work and finish off our chances."
AS Nancy, beaten 1-0 at Sochaux on Saturday, fell into the third relegation place while former Champions League finalists Monaco are among five clubs just a point better off after a 1-1 draw at Europa League hopefuls St Etienne.