Arsenal have replaced Liverpool at the top of the English Premier League, after the Gunners beat West Ham 3-1 and the Merseysiders lost 2-1 to Manchester City.
The Gunners had fallen behind at West Ham on Thursday when goalkeeper Wojciech Szczesny spilled Kevin Nolan's shot and Carlton Cole pounced to score a minute into the second half.
West Ham goalkeeper Adrian was also culpable for Arsenal's equaliser, with Walcott cutting in from the right and striking between his legs in the 68th.
Walcott headed Arsenal in front three minutes later, and Lukas Podolski rounded off the win in the 79th at Upton Park to ensure Arsenal's three-game winless run in the league ended, as West Ham dropped into the relegation zone.
"It's great to be top but it's going to be a very tough league this year," Walcott told BT Sport.
"Everyone is very tight, a lot of people are taking a lot of points off each other so we've just got to make sure we look after ourselves."
Manchester City ended Liverpool's short stay at the top of the Premier League and moved ahead of their title rival after Alvaro Negredo secured their win.
England's most potent attacking team quickly recovered after Phillippe Coutinho put Liverpool ahead in the 24th minute.
Vincent Kompany headed City level seven minutes later and Negredo's high shot was fumbled into the net by Liverpool goalkeeper Simon Mignolet on the stroke of half-time.
Liverpool manager Brendan Rodgers could face an English Football Association charge for calling into question the referee's integrity and implying potential bias.
"They were horrendous," Rodgers said.
"We never got any decision."
Aggrieved that Raheem Sterling had a goal ruled out for offside and Luis Suarez was denied a late penalty, Rodgers questioned why a referee from the Manchester area, Lee Mason of Bolton, was given a match involving a Manchester team.
City ended the day as they started: a point behind the leaders, but Liverpool dropped to fourth as a four-match winning run ended.
Chelsea are now two points off the lead after Eden Hazard scored in the 28th minute to give the west London club a 1-0 win over Swansea.
Meanwhile, Manchester United have moved up to sixth following their fightback at Hull.
Hull scored twice inside the first 15 minutes, but David Moyes's side were level inside 25 minutes. Wayne Rooney's 150th Premier League goal making it 2-2.
In the second half, Hull defender James Chester, who'd earlier opened the scoring for his team, headed in an own goal that gave United the 3-2 win.
Everton's hopes of completing 2013 unbeaten at home in the Premier League vanished as they lost 1-0 at home to bottom club Sunderland after keeper Tim Howard was sent off.
Everton are fifth with 34 points, one more than Newcastle United who thumped nine-man Stoke City 5-1.
Newcastle are closing in on the top four after a 5-1 thrashing of stoke.
Tottenham Hotspur are eighth following a 1-1 draw with West Bromwich Albion in Tim Sherwood's first match in charge since being named as the club's permanent manager.
Wins for Crystal Palace at Aston Villa and Fulham at Norwich City meant the three sides that began the day in the bottom three all won, leaving West Ham to drop into the bottom three.
Cardiff City were beaten 3-0 at home by Southampton, raising fresh doubts over the future of manager Malky Mackay.