Poland moved top of the group after the game at the National Arena in Bucharest was spoiled by crowd violence that forced the Slovenian referee, Damir Skomina, to interrupt the game on two occasions.
Unbeaten Poland lead Group E with 10 points from four matches, followed by Montenegro on seven. Montenegro suffered their first defeat in the group, squandering a two-goal lead to lose 3-2 in Armenia.
Grosicki collected the ball in the centre circle, weaved his way past five Romania defenders and gave the keeper, Ciprian Tatarusanu, no chance with a powerful shot for his ninth international goal after 11 minutes.
Vlad Chiriches came closest to scoring for the home side when Poland’s keeper, Lukasz Fabianski, parried his close-range effort in the 32nd minute.
Lewandowski almost doubled Poland’s lead when his curling free-kick went just wide soon after the break. The Bayern Munich striker, however, made amends seven minutes from time when he put the result beyond doubt with a cool finish.
Lewandowski completed the rout when he converted a penalty in stoppage time for his seventh goal in the qualifying campaign.
The match was interrupted for a minute at the end of the first half after fans threw flares and firecrackers on to the pitch.
The ugly scenes continued early in the second half when Lewandowski fell to the ground but escaped unhurt after a firework, thrown by home supporters, landed near the striker.
Romanian police detained six Polish fans for violence before the game.
Romania, who lost for the first time under their German coach, Christoph Daum, are fourth in the standings with five points.
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