Ivory Coast striker Gervinho appeared to go down easily in the area near the end of the game, prompting Newcastle midfielder Joey Barton to grab his shirt and pull him up.
Gervinho then slapped Barton, who fell over theatrically and was lucky his involvement in the scuffle did not earn him more than a yellow card at St James' Park.
Arsenal manager Arsene Wenger questioned whether the referee had even seen the incident and said both players should have received the same punishment.
"Gervinho didn't deserve a red card. He [referee Peter Walton] gives a red to the two or a yellow to the two," he told ESPN television.
"The referee has not seen it and the linesman has not seen it properly. I'm 100 percent sure the referee didn't see the incident. I would like to see who made this decision."
Arsenal's fellow title hopefuls Liverpool also dropped points in a 1-1 draw with a plucky Sunderland side at Anfield.
Liverpool took the lead with Luis Suarez's 12th-minute header but the Uruguayan's earlier penalty miss and a disallowed Andy Carroll effort left Sunderland in the game.
A great volley by winger Sebastian Larsson in the 57th minute gave the Swede a goal on his debut and the visitors a share of the points.
"We might have had a few more chances and were a wee bit unlucky in getting Andy Carroll's goal knocked off, which would have put us 2-0 up," Liverpool manager Kenny Dalglish told reporters.
"But Sunderland made it difficult for us, they worked really hard and I think they deserved a point."
QPR got a sharp lesson in top-flight football with a 4-0 thrashing by visitors Bolton Wanderers while fellow promoted side Norwich City snatched a 1-1 draw at Wigan Athletic.
Wolverhampton Wanderers won 2-1 at Blackburn Rovers and Fulham drew 0-0 with Aston Villa in the day's other games.
Before their match turned ugly, Arsenal dominated possession with their attractive passing game although without midfielders Cesc Fabregas and Samir Nasri, poised to join Barcelona and Manchester City respectively, they lacked a decent final ball.
But a reasonable performance will be forgotten in the light of the melee that erupted some 15 minutes from time.
At the centre of the skirmish was the often controversial Barton, who had been a surprise inclusion in the Newcastle team after being transfer-listed earlier this week.
His fiery temper had already been tested earlier in the game when his leg was stamped on by Arsenal's Alex Song, in an incident the referee appeared not to see, and he had remonstrated with the fourth official.
After dragging Gervinho to his feet, Barton received a slap from the striker but was seen telling the referee he had been "punched."
Gervinho was given his marching orders while Barton remained on
