McGregor had made saves from Daniel Majstorovic, Charlie Mulgrew and Emilio Izaguirre before Celtic substitute Anthony Stokes, on for Gary Hooper, won a spot-kick inside the final 10 minutes.
The Scotland number one dived to his left to save the Greece international's effort to keep second-placed Celtic a point behind with a game in hand.
The game took place against a background of heightened security after a parcel bomb was recently sent to Lennon.
However, despite calls for calm, there was still a deafening din inside Ibrox at kick-off time.
Celtic had a penalty claim in the seventh minute when Joe Ledley clashed with Ibrox full-back Steven Whittaker in the box but referee Craig Thomson was unimpressed, no less so than Lennon who turned to gesture in frustration to the main stand.
Moments later, Light Blues striker Kyle Lafferty nearly got on the end of a pass from fellow Northern Ireland international Steven Davis as Celtic keeper Fraser Forster and Charlie Mulgrew almost got in to a tangle.
Then, after Rangers' on-form striker Nikica Jelavic had set him up with a chest pass, Maurice Edu's shot from the edge of the box got deflected past for a corner which led to a spell of pressure which Celtic survived.
In the 18th minute, another orchestrated show of support for Lennon from the away fans in the Broomloan Road stand came as Rangers continued to press for the opener.
After Samaras pushed Rangers skipper David Weir to the ground inside the home side's penalty area, during an altercation which followed the Greece international's challenge on McGregor, both players were booked.
The champions edged possession but in the 26th minute, Hoops skipper Scott Brown sent a left-footed drive from 25 yards past McGregor's left-hand post, moments before Steven Naismith's angled-volley from 12 yards missed the target at the other end.
By the time Celtic striker Hooper flashed a drive from the edge of the box high over the bar, the visitors had come back into the game.
Two minutes into the second half, Weir headed over a Davis corner before a Naismith volley from the edge of the box was deflected for another corner which came to nothing.
Rangers should have scored in the 51st minute when Gregg Wylde's cross from the left was met by Lafferty but the lanky striker somehow headed past the post from six yards.
Seconds later, at the other end, Hoops midfielder Beram Kayal drove inches wide from 20 yards.
Ledley had to come off in the 54th minute after being fouled by Naismith wide on the left and was replaced by Kris Commons.
The Light Blues' defence struggled to clear their lines following the resulting free-kick and needed McGregor to make a great save from Majstorovic's close-range header to keep the game goalless.
Kayal was booked in the 61st minute for fouling Jelavic 30 yards from goal as the Rangers striker broke with purpose after turning Majstorovic on the halfway line and Lafferty fired the free-kick wide.
As the tension increased, Naismith had a left-footed effort from 20 yards which just sped wide before a Mulgrew header from Commons' corner forced another save from McGregor.
In the 75th minute, the Rangers keeper made yet another great save, this time from Izaguirre's shot after the Hoops left-back combined well with Samaras inside the Light Blues' penalty area.
There was great drama in the 81st minute when Celtic were awarded a contentious penalty when Celtic substitute Stokes clashed with Davis inside the Rangers penalty area.
Samaras, who had scored a penalty in the 2-0 win at Ibrox at the start of the year, saw his spot kick brilliantly saved by McGregor who dived to his left to push the ball round the post.
There was time for Stokes to be booked for diving having gone to ground in the box after going past Rangers substitute John Fleck.