The former NBA player - who was married to Khloe from 2009 until 2013 - was hospitalised on Tuesday, 13 October after he was found unresponsive in a Nevada brothel at around 3pm in the afternoon.
His condition is not known.
According to multiple reports, Khloe was in Las Vegas when news of her ex-husband broke, and she immediately rushed to his bedside at the Sunrise Hospital. It's now been reported that her mother Kris Jenner and elder sister Kim Kardashian are also with her.
Brother Rob Kardashian is also thought to be en route.
A source told People: "Everyone has dropped everything to get there. There are a lot of calls and texts going back and forth as everyone is figuring out how they can help and be there for each other. They are asking their close friends to pray for Lamar, for healing and strength.
"Everyone is very upset. They know that Lamar has his demons, but the fact is that he was a very important part of Khloé's life for so long, and she's very upset about this. They all are."
Lamar was found unresponsive at the brothel by two women. Paramedics were called and he was taken by ambulance to a nearby hospital, where he was treated before being transferred to Sunrise.
Dennis Hof, the owner of the brothel, has said Lamar checked in on Saturday, 10 October and had seemed "happy and healthy". He claimed the basketball star took herbal Viagra while on the premises and ordered a bottle of Cognac.
A statement issued by Lamar's rep to Us Weekly today said: "Family and friends are extremely concerned for Lamar. Please don't listen to the false information being circulated unofficially. Please respect the family's privacy during this difficult time."
While Lamar and Khloe split in 2013, their divorce papers were not finalised until earlier this year and they had remained in contact.
On the most-recent episode of Keeping Up With The Kardashians, viewers saw Khloe chatting on the phone with her former husband, much to the dismay of sister Kim who worried about their continued contact.