Oxford are second, four points behind Chesterfield, after they recorded a 3-1 away win at struggling Hartlepool, with goals from on-loan Fulham teenager Ryan Williams, Dean Smalley and an injury-time penalty from Asa Hall.
Luke James scored for the home side, who had Steve Howard sent off late on for a professional foul on Scott Davies that gave Oxford their spot-kick. For full coverage of all today's fixtures visit the ITV Sport website here
Jed Wallace opened the scoring in the 20th minute, David Connolly made it two six minutes later and, after Andy Wright had been sent off, Patrick Agyemang wrapped up a 3-0 victory six minutes before the break.
Southend have made their best start to a season in 23 years and share top billing with Oxford.
The Shrimpers defeated Northampton 2-0 at Roots Hall today thanks to Anthony Straker (38) and Freddy Eastwood (62).
Oxford lead the table on goal difference after a 3-1 victory over Torquay.Striker James Constable was instrumental in Chris Wilder's side claiming their third league win in a row, scoring a superb 62nd-minute goal and laying on the third for fellow substitute Asa Hall (78).
Portsmouth suffered a sorry start to life in SkyBet League Two as they crashed to a 4-1 home defeat at the hands of Oxford. Pompey are considered one of the favourites for promotion and things looked promising for Guy Whittingham's relegated side.
They took a 25th-minute lead through Patrick Agyemang in front of a league-record 18,000 strong crowd at Fratton Park.
However, Oxford had not read the script and they turned things round to go into the break 2-1 up with goals from Deane Smalley and Alfie Potter.
Things got worse for Portsmouth just before the hour-mark when they lost Johnny Ertl to a red card and Oxford made the most of their numerical advantage to add salt to their opponents' wounds with further goals from Potter and Smalley.
For a full round-up of all today's action click here for in-depth match coverage from ITV Sport
Thames Valley Police say they're taking a zero tolerance to football violence after three more men were arrested after clashes between rival Oxford United and Bristol Rover fans. In total six men have been arrested after the match at the Kassam Stadium on February 9.
Supt Christian Bunt, said: “I will not tolerate football related disorder and I expect further arrests to be made. We cannot allow a small number of individuals to threaten the safety and enjoyment of attending a football match.
"We will also be taking a zero tolerance approach on match days where we believe trouble is likely.”
Thames Valley Police have made three more arrests after fans of Oxford United and Bristol Rovers clashed during a mathc at the Kassam Stadium on Saturday, February 9. A 47-year-old man from Marston, a 40-year-old man from Wantage and a 39-year-old man from Witney are all in police custody.
They were arrested on suspicion of committing public order offences. Three people were arrested at the match. Two men, both aged 47, have been released on bail until March 8and a third man, aged 45, has been bailed until March 9.
Three men arrested at the Oxford United and Bristol Rovers football match on Saturday 9th February have been released on bail.
They were arrested on suspicion of committing public order offences. Two men - both aged 47 - have been released on bail until 8th March, while the third man who is 45-years-old, has been bailed until 9th March.
“There is an ongoing post-match investigation to determine the circumstances surrounding what happened. There was a substantial policing operation in place to ensure the game ran smoothly and it is a shame that a small minority of supporters seemed to be intent on causing trouble.”
– Area Commander, Supt Christian Bunt, Thames Valley Police