According to BBC Sport, scuffles broke out and flares were lit during the 1-1 draw, which saw Liverpool advance 3-1 on aggregate to the quarter-finals.
Sky Sports News HQ's Amy Lewis provided details of the individual charges for both clubs:
MUFC & LFC charged by UEFA for several counts. Utd's inc crowd disturbances & throwing objects. LFC's inc late kick-off & illicit chants3/18/2016, 9:47:16 AM
Sky Sports News HQ gave details of the hearing:
Sky Sports News HQ @SkySportsNewsHQ
BREAKING: Case to be heard by UEFA's control, ethics and disciplinary body on May 19. #SSNHQ3/18/2016, 9:51:15 AM
Fighting broke out in Old Trafford's East Stand when a small group of visiting supporters were revealed to be sitting among the United fans.
BBC 5 Live commentator Ian Dennis described the incident:
Punches are being traded between rival supporters. ... I have seen three red seats from the Liverpool section being thrown into the Manchester United section. There are about 10 Liverpool supporters sat on the front row of the top tier and there is a human wall of police officers in luminous clothing protecting the Liverpool fans sat in the Manchester United end.
The radio station also provided images as a flare was set off in the stadium:
BBC 5 live Sport @5liveSport
Official term: 'scenes' Second half under way - can #MUFC stage a come back? #MUFCvLIV ⚽📻 https://t.co/NI1sjtpg98 https://t.co/5GiTG6shzT3/17/2016, 9:20:07 PM
As noted by BBC Sport's report, UEFA places the responsibility on the host club to keep order within the stadium—and are liable as such—but all clubs can be sanctioned for "inappropriate behaviour on the part of their supporters."
Given the fierce rivalry between the two clubs, tensions were already raised within the stadium and boiled over into some unpleasant scenes.
Jack Gaughan, Oliver Todd and Chris Wheeler of MailOnline relayed a Greater Manchester Police spokesman's comments explaining the situation:
Just prior to the full-time whistle of the Manchester United versus Liverpool match, police were made aware of a physical altercation between supporters from both sides in a small section of the crowd. Police and stewards broke up the altercation and inquiries are continuing with those involved.
The United fan was arrested for being drunk and disorderly. He remains in police custody for questioning. He was arrested outside the ground. Of the four Liverpool fans, two men were arrested for public order offence of affray outside the ground and they are still in police custody. One man was arrested for criminal damages for smashing the windows of a minibus and is still in police custody. The fourth man was arrested for letting off a flare inside the ground. He was cautioned and released.
Philippe Coutinho cancelled out Anthony Martial's penalty just before half-time to effectively seal Liverpool's spot in the last eight as it meant United needed to score three more goals, per BT Sport Football:
BT Sport Football @btsportfootball
.@Phil_Coutinho's beautiful finish sends @LFC on their way to the #UEL quarters... https://t.co/ISHkDXNlhu3/18/2016, 7:15:06 AM
The Reds will discover their quarter-final opponents in Friday's draw.