London - Manchester City moved level on points with English Premier League leaders Arsenal following a comprehensive victory at Upton Park inspired by Yaya Toure.
Toure opened the scoring with a powerful drive and with 17 minutes remaining his shot rebounded off the post and went in off West Ham goalkeeper Robert Green.
Adam Johnson, on as a substitute, added a third goal before James Tomkins pulled a goal back for the hosts.
Victory for City came without skipper Carlos Tevez as the Argentina striker was suspended for picking up five bookings this season.
It meant there was no warm homecoming reception for the player who played a key role in saving West Ham from relegation in 2007.
The mention of Tevez was a reminder of how desperate West Ham's position has become at the foot of the table, despite a recent victory over Wigan.
Scott Parker has been the player they have turned to for inspiration this time around.
He had an early volley flash wide and set a chance up for Junior Stanislas that was saved, but City provided a greater threat in the first half.
Brazilian striker Jo, Tevez's replacement, forced Green into an early low save, then he crossed from a similar position on the left channel but Mario Balotelli glanced his finish wide.
Midfielder Toure had already had a shot saved by Green after a powerful run past three defenders, then he broke the deadlock after 30 minutes.
Gareth Barry set up the goal, although his pass into the penalty area from the left was originally intended for David Silva, with the Spaniard running ahead of the ball.
It caught out the West Ham defence and Toure, who escaped the attentions of Jonathan Spector, took the ball into his stride and his finish flew past Green and into the roof of the net for this third of the season.
Balotelli could have added another when he was left unmarked from a Silva corner, but it needed Pablo Barrera to hack off the line.
West Ham's response was a shot from Parker that flew wide from 25 yards, and a header from Frederic Piquionne that that just cleared the crossbar.
In the second half, Balotelli was booked for dissent. Mancini took the Italian off as it looked like he was poised to get sent off, with the forward heading straight down the tunnel.
It meant a row with a player in consecutive weeks for Mancini, with Tevez upset at being taken off last weekend.
The hosts thought they had a decent penalty shout when Toure handled in the area - but referee Phil Dowd waved play on.
And Toure was celebrating again with 17 minutes left when he breezed past Tomkins on the left before closing in on goal.
His finish hit the post and went in off Green so it was an own goal for the keeper.
Johnson added the third when Silva sent him through to round the keeper and finish, with Tomkins goal a header from a corner.