Sunderland led through an early goal from Aaron Connolly just 10 minutes into the game before the match was put on hold to deal with a number of separate incidents.
In the closing moments of the first-half, Millwall's Ryan Leonard went over to take a throw-in and immediately called for medical assistance.
It has been reported that there were two separate medical emergencies in the Millwall end and a third occurred in the Sunderland away section.
Medical staff from both teams were on hand and rushed over to help before the players left the pitch, led by the referee after they were waiting in the technical area.
Millwall's staff stood down as one affected supporter was lifted onto a stretcher to the crowd's applause.
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Both clubs announced on X, formerly Twitter, that the match had been suspended after both sides left the field of play.
Millwall posted: "The players have left the pitch following a medical emergency in the crowd. We send our best wishes to those receiving attention."
Sunderland shared a similar statement: "Play has been temporarily suspended due to a medical emergency at The Den."
The players eventually returned to the pitch and the match resumed after a delay of around 30 minutes.
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Speaking to talkSPORT, Millwall boss Neil Harris said: "I have huge pride. For the doctor and physio to respond so quickly showed real bravery.
"We take for granted when people do their jobs but the way they reacted was just immense.
"It was the Sunderland doctor as well - the way he ran into the home end to treat a lady and react like that, you can't praise people like that enough.
"I'm a huge fan of the NHS and have family members who work for the NHS and I know the work they do - it's mega work and that's why we stay safe.
The response of those individuals was very telling today and so I express on record real praise for them."
Despite the disruption, Regis Le Bris' side looked like good value for the win at the Den.
Sunderland brought 3,000 fans with them for the trip to south London, knowing that a win or a draw would put them back at the summit of the Championship table.
Coventry held the Black Cats' title rivals Sheffield United to a 2-2 draw earlier on Saturday, allowing Sunderland to hold their own fate in their hands.
Despite a late effort from Femi Azeez, a 1-1 draw was still enough to keep the Black Cats on top.
Meanwhile a point means that Millwall stay in eighth position.