A header from striker Benjani denied Manchester United their first win of the league campaign at Portsmouth as both sides ended with 10 men.
Ronaldo was sent off after clashing with Hughes in the second half
United took the lead when new signing Carlos Tevez set up Paul Scholes who fired in from 25 yards.
Portsmouth equalised when Benjani sent a superb header past Edwin van der Sar.
Sulley Muntari earned a second yellow for a late tackle on Michael Carrick, while Cristiano Ronaldo saw red for his part in a fracas with Richard Hughes.
All eyes were on Tevez who was thrust to centre stage by manager Sir Alex Ferguson as a result of Wayne Rooney's injury.
In one sense, it worked out well for the Scot because it postponed the inevitable scrutiny over whether Rooney and Tevez could play in the same team together because of their similar styles.
Of course, Ferguson would prefer to have that enviable problem, but while Rooney is sidelined he can sit back and marvel at the industry and talents of his new Argentine signing.
There was not too much to marvel at after a quiet, yet physical, opening few minutes to his United career, but then Tevez showed a bit of the Argentine flair those who have followed his progress knew he had.
The 23-year-old collected the ball on the edge of the area with his thigh before turning to lay a pass into the path of Scholes.
The former England ace, lethal from most distances, smacked the ball from 20 yards past David James, the keeper doing his best to pull off a miracle save, to no avail.
Aside from that piece of action there was little else to talk about from an opening half that exuded a lot of energy but little substance.
United should have extended their lead straight after the break but Nani's low shot from eight yards was brilliantly blocked by the feet of James, before Tevez's control and volley from eight yards flew over the Portsmouth bar.
Benjani salvaged a point for Pompey
A 2-0 deficit would have been slightly unfair on the home side, whose line-up altered when boss Harry Redknapp brought on Matthew Taylor and Djimi Traore.
Redknapp's tactical substitution did the trick when Taylor found Benjani from the right and the Zimbabwean gave Van der Sar no chance with a cracking header.
United, who drew with Reading in their first match, were desperate for their first three points of the campaign and both Giggs and Ronaldo went close only to be denied by the alert James.
At the other end, Portsmouth striker David Nugent was well marshalled by Rio Ferdinand and Nemanja Vidic, but the one time he managed to find himself one-on-one with Van der Sar, the former Preston star slipped embarrassingly.
The match ended on an unsavoury note when Muntari received his marching orders from referee Steve Bennett after a late challenge on Carrick, and then Ronaldo received a straight red for his part in a scuffle with Hughes.
And the red cards late in the game overshadowed a match that had earlier been all about Tevez's debut.
Portsmouth coach Joe Jordan:
"We ended the game with four left-footed players in defence.
"We hung on in there after being put under a lot of pressure and we showed a lot of character.
"The couple of substitutions we made at half-time changed things for us in a positive way after the break."
Manchester United assistant Carlos Queiroz:
"We played much better than we did against Reading and created lots of opportunities.
"We controlled the game in the first half but Portsmouth put more pressure on us in the second half and we lacked a bit of composure.
"Defensively, we did not look on Benjani's goal."
Portsmouth: James, Pamarot (Hughes 64), Cranie (Traore 46), Distin, Hreidarsson, Utaka, Pedro Mendes (Taylor 46), Davis, Muntari, Mwaruwari, Nugent.
Subs Not Used: Ashdown, Kanu.
Sent Off: Muntari (83).
Booked: Muntari, Davis.
Goals: Mwaruwari 53.
Man Utd: Van der Sar, Brown (Eagles 88), Ferdinand, Vidic, Evra, Ronaldo, Carrick, Scholes, Nani, Giggs (O'Shea 81), Tevez.
Subs Not Used: Kuszczak, Pique, Fletcher.
Sent Off: Ronaldo (85).
Goals: Scholes 15.
Ref: Steve Bennett (Kent).