Injury-ravaged Liverpool looked good for a draw at home to the champions when Mellor, included in the team because of injuries to Djibril Cisse, Milan Baros and Luis Garcia, lashed home to sentence Arsenal to only their second defeat in 55 league games.

His goal meant the Gunners remain five points behind Barclays Premiership leaders Chelsea.

Liverpool had led in the first half through an exquisite Xabi Alonso strike. Patrick Vieira equalised but then gave more sustenance to Chelsea’s title ambitions by fouling Alonso and getting booked, meaning he misses the crucial league game against Jose Mourinho’s men next month.

Liverpool produced a display of effort, commitment and character which will gladen the heart of worried boss Rafael Benitez, who had admitted he had endured some sleepless nights because of his club’s injury problems.

Benitez shuffled his depleted pack, bringing in youngster Florent Sinama-Pongolle in attack while Alonso and Harry Kewell returned to the starting line-up after the Champions League defeat in Monaco.

Arsenal had Thierry Henry and Jose Antonio Reyes fit while Dennis Bergkamp was out with an Achillies problem.

Liverpool started with Pongolle on the right and with Steven Gerrard, returning for his first game at Anfield for over two months following a broken foot, supporting Mellor through the middle.

The England midfielder’s drive and leadership was critical to his patched-up team from the start.

After just two minutes he surged into the box and, after cutting inside Kolo Toure, tumbled after what most Liverpool fans claimed was a trip by the Gunners defender. Referee Alan Wiley thought otherwise.

Arsenal tried to draw the sting from an impassioned home side as they slowed the game and played the ball around.

Alonso saw a 20-yard shot curl well wide and Gerrard sent in a free-kick that deflected off the wall for a corner, from which Dietmar Hamann’s drive was also blocked.

However, all too often when the ball was played into Mellor, despite his honest toil he rarely maintained possession. Harry Kewell was not much better with the Australian a long way short of his top form.

Arsenal were tentative but Thierry Henry, Reyes and Robert Pires threatened to dismantle the home side on a couple of occasions.

Undaunted, Liverpool drove forward again and Steve Finnan sent a curling cross to the far post from which Kewell’s header was plucked out of the air by Lehmann. Then an Alonso corner was met by Sami Hyypia eight yards out but his header cleared the bar.

By now, Liverpool’s hounding and running had knocked Arsenal out of their stride. It was far from pretty but the home side were growing in confidence.

And their efforts were rewarded after 41 minutes when they stormed into the lead. Finnan’s cross-field ball was nodded down by Kewell and Gerrard hesitated before threading it through into the path of Alonso, who steered his shot high into the net.

Once Liverpool had weathered the opening few minutes of the second half, they began to threaten again. Gerrard was leading by example while even Kewell was starting to win tackles.

A string of Gerrard corners, plus some neat control from Sinama-Pongolle kept Arsenal on the back foot. Then Hamann motored onto a pass 25 yards out to force Lehmann into a save to his left.

But Arsenal are at their most dangerous in such circumstances, with teams coming onto them thinking they are in control.

And after 57 minutes the visitors put together a sustained passing move which included Lauren and Henry and ended with Pires guiding the ball into Vieira’s path in the box.

The Arsenal skipper deftly lifted the ball over Chris Kirkland’s dive and into the net for a stunning equaliser. A minute later, though, Vieira spoiled his good work when he clattered into Alonso and was booked.

Arsenal brought on Robin van Persie for Reyes as Liverpool visibly tired. The home side introduced Antonio Nunez in place of Sinama-Pongolle, the former Real Madrid man making his first appearance for the club after suffering a knee injury in pre-season.

Vieira then went down with Hamann not even having made contact – an act worthy of a booking, and thus a sending-off, but he escaped.

Gerrard forced his way down the left to see a shot parried by Lehmann, and then went even closer when Mellor and Nunez had battled to get the ball into the box. Gerrard latched on to the ball and forced Lehmann into a save to his right.

Six minutes from time Henry was booked for following through to leave Alonso on the floor with tension running high.

Arsenal’s movement was stretching Liverpool in all directions, and Henry almost jinked his way into the box only for Kirkland to snatch the ball off his toes.

But suddenly Arsenal were beaten when, in injury-time, Mellor found space after Kirkland’s clearance and slammed the ball past the despairing dive of Lehmann to secure a famous win for Liverpool.