A Contraversial decision by the officials almost sparked a riot as Mansfield snatched a win.
Gus Unlenbeek's deflected effort was the only real chance in a dull, goalless first half.
Oxford's Tcham N'Toya forced a good save from Kevin Pressman after 56 minutes, and Danny Reet shot straight at visiting keeper Andrea Guatelli five minutes later.
The deadlock was broken after 84 minutes when Unlenbeek's cross was headed across the box by Ritchie Barker and Reet rose to head it in.
Guatelli clawed at the effort and appeared to make a good save, but the linesman gave a goal and the Italian goalkeeper was then booked for contesting the decision.
The game was then stopped for five minutes as rival fans tried to get to each other, but the goal stood and the Stags took the points.
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Oxford Mail, by Jon Murray on 02 April 2006
CROWD trouble marred Oxford United's game at Mansfield on Saturday, causing a five minute stoppage in play late in the second half.
The depressing spectacle of football hooliganism returned, and it appeared as though Oxford fans were mostly to blame.
Two ran onto the edge of the pitch before being rugby-tackled by stewards who apprehended them until police arrived.
There were several other instances of United fans jostling stewards aggressively.
However, there were claims later that the trouble began after a bottle was hurled into the Field Mill Ground's North Stand, which was housing more than 700 Oxford supporters.
Police are studying CCTV pictures and photographic evidence before deciding whether to make further arrests.
But the Football League seem certain to investigate, with the incidents inevitably included in the referee's match report.
Clashes started soon after Mansfield scored from a controversial 84th-minute goal, which proved to be the winner.
Danny Reet's header at the far post was clawed out of the goal by U's goalkeeper Andrea Guatelli, but the linesman flagged that the ball had crossed the line.
The 1-0 defeat was Oxford's first for five games, and first under new boss Jim Smith.
United's preparations had been badly hit by a flu bug which ruled out Andy Burgess, Matt Robinson and, late in the day, Jon Ashton.
Then Leo Roget felt unwell during the warm-up and had to be replaced by John Dempster, leaving Oxford with just four on the bench.
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Mansfield CHAD:
Uhlenbeek's cross was headed back across goal by Barker and Reet applied a close range headed finish that the officials decided had crossed the line before keeper Guatelli got his hands behind it to push out.
The visitors were incensed and as the arguments on the field continued a large number of Oxford fans piled over the barriers towards the home fans and there were fights with home stewards.
The restart was held up as the stewards tried to restore order.
But the final whistle sparked more trouble at the away end with fans also fighting outside in the car park to bring an unsavoury end to a strange afternoon.