With title rivals West Ham not in action, the Saints went two points clear at the summit having played two more matches.
Jos Hooiveld got the ball rolling in the 15th minute before second-half strikes from Aaron Martin, Adam Lallana and Tadanari Lee.
New Leeds manager Neil Warnock was on hand to watch the club come from two behind to claim a dramatic 3-2 win against Doncaster in Neil Redfearn's final match as caretaker boss.
Mamadou Bagayoko scored in each half to give Rovers control of the match, but Andros Townsend hit back within a minute of the second and Adam Clayton levelled 10 minutes from time. Luciano Becchio then struck nine minutes into injury-time to secure the points.
Nottingham Forest moved off bottom spot and to within three points of safety with a 2-0 defeat of fellow relegation battlers Coventry at the City Ground. Forest have struggled for goals all year but first Garath McCleary (74) and then Robbie Findley (86) found the target against the Sky Blues.
Fourth-placed Cardiff were brushed aside with surprising ease by Ipswich, who celebrated a 3-0 win at Portman Road. Lee Martin scored in each half and Michael Chopra netted three minutes after the interval to down the Bluebirds.
Peterborough also won 3-0 against visiting Bristol City, who now sit fourth bottom. Lee Tomlin (7, 84) started and finished the scoring, with David Ball notching after 62 minutes.
Crystal Palace romped to a 4-0 home win over Watford, Chris Martin scoring twice. Wilfried Zaha had broken the deadlock in the 22nd minute and Kagisho Dikgacoi rounded off the scoring just after an hour's play.
Portsmouth, who this week entered administration again, had Dave Kitson sent off for two cautions after 42 minutes but held on with a man down for over half an hour against Barnsley. Jim O'Brien eventually made the numbers count in the 76th minute and Matt Done made it 2-0 in the closing seconds.
Source: PA
Source: PA