Lionel Messi hit a hat-trick as Barcelona thrashed Manchester City 4-0 at the Nou Camp to seal a nightmare return for former boss Pep Guardiola.
Guardiola watched his side crumble in their Champions League Group C clash after the 53rd-minute sending-off of another former Barcelona star, keeper Claudio Bravo, for handling outside his box.
Messi had opened the scoring in the 17th minute and went on to complete his hat-trick just past the hour-mark before Neymar - who earlier missed a penalty - added a late fourth.
Barcelona were themselves forced to play out the last 17 minutes with 10 men following the sending-off of substitute Jeremy Mathieu for two bookings in quick succession.
Celtic's hopes of building on last month's epic 3-3 draw with City were blown apart as Borussia Monchengladbach cruised to a 2-0 win at Parkhead.
Lars Stindl opened the scoring in the 57th minute and Andre Hahn grabbed the second in the 77th minute - and but for a fine display from Hoops keeper Craig Gordon it could have been more.
Mesut Ozil scored a second-half hat-trick as Arsenal eased past Ludogorets 6-0 at the Emirates Stadium to maintain their slender lead at the top of Group A.
A brilliant lob from Alexis Sanchez paved the way for an easy night for the Gunners, for whom Theo Walcott and Alex Oxlade-Chamberlain also found the net.
Arsenal will now look ahead to a potential group decider against Paris St Germain next month after the French side beat Basle 3-0 to stay level on seven points.
Angel Di Maria gave the hosts the lead and Lucas Moura added a second before Edison Cavani wrapped up victory with a last-minute penalty.
Yannick Carrasco scored the only goal of the game as Atletico Madrid maintained their 100% record in Group D with a 1-0 win at Rostov.
In-form Carrasco's 62nd-minute volley from a Juanfran cross proved enough to give Atletico the points in a game they largely dominated with Fernando Torres missing a number of fine chances.
Also in Group D, Bayern Munich won for the first time in four games as Thomas Muller's 13th-minute effort set up a comfortable 4-1 triumph over PSV Eindhoven.
Joshua Kimmich nodded home his side's second on 21 minutes and despite Luciano Narsingh temporarily reducing the deficit, second-half strikes from Robert Lewandowski and Arjen Robben relieved some of the pressure on boss Carlo Ancelotti.
Group B leaders Napoli saw their 100% record brought to an end as two Vincent Aboubakar goals helped Besiktas to a surprise 3-2 win in Italy.
Besiktas 3-2 Napoli. Should've been disallowed for offsides. pic.twitter.com/pU15988Nkc— Serie A News (@TransfersCalcio) October 19, 2016
Adriano Correia opened the scoring for the visitors and despite an equaliser from Dries Mertens, Aboubakar's opening effort gave the Turkish side the half-time lead.
Napoli equalised for a second time through Manolo Gabbiadini's penalty but Aboubakar's header four minutes from time sent the Turkish side up to second.
Meanwhile the pressure continued to mount on Dynamo Kiev boss Sergei Rebrov after Eduardo Salvio's early penalty and a second-half effort from Franco Cervi gave Benfica a 2-0 win in Ukraine.