Arturo Vidal scored twice in the first eight minutes as rampant Juventus took advantage of a slip-up by second-placed AC Milan, who needed a last-minute goal from Zlatan Ibrahimovic to snatch a 1-1 draw at home to Bologna.
The tussle at the top was overshadowed by events at relegation-threatened Genoa where supporters, furious after seeing their team go 4-0 down at home to Siena, threw smoke bombs onto the field and caused the referee to halt the game.
Some ultra fans also climbed over the perspex barrier around the pitch and demanded the players take off their shirts after chanting they were unfit to wear them.
Several players did so and the game restarted with Siena, who had retreated to the dressing room, eventually winning 4-1.
The scenes happened just a week after Italian football came together following the death of Livorno's Piermario Morosini after a cardiac arrest on the pitch.
Vidal's quickfire brace had Roma floundering, the Chilean midfielder turning in Paolo De Ceglie's low cross after four minutes and rifling in the second with an angled shot after Mirko Vucinic laid the ball into his path.
Embarrassingly outclassed, visitors Roma were 3-0 behind and reduced to 10 men after half-an-hour.
Goalkeeper Maarten Stekelenburg got the faintest of touches as Claudio Marchisio flicked the ball past him and went tumbling over, the referee ruling that the Dutchman was the last man and had to go.
Andrea Pirlo's penalty was saved by replacement goalkeeper Gianluca Curci but the Juventus playmaker followed up to score from the rebound.
Marchisio made it 4-0 early in the second half before Juventus eased off.
Gaston Ramirez put Bologna ahead at San Siro and Milan's problems deepened when Daniele Bonera was sent off in the 82nd minute for a second bookable offence.
However, Bologna had lost their composure by then and Ibrahimovic levelled with his 24th Serie A goal of the season, scoring with a first time shot after Urby Emanuelson's cross found him at the far post.
Antonio Nocerino and Robinho were both close to last-gasp winners for Milan, who have taken only one point from their last two games.
LAZIO DRAW
Juventus, chasing their first scudetto since they were stripped of the 2005 and 2006 titles in the Calciopoli match-fixing scandal, have 71 points from 33 games while Milan have 68 with five matches each to play.
"No one thought we would get just a point from two home games but now we have to get back to winning ways at home and after that we'll see," said Milan coach Massimiliano Allegri, referring to the 2-1 defeat by Fiorentina earlier this month.
Third-placed Lazio, who have 55 points and are in the Champions League qualifying round spot, conceded an injury-time equaliser in a 1-1 draw with relegation-threatened Lecce.
