Hollywood legend Bruce Willis has spoken for the first time since his devastating dementia diagnosis.

Willis was the star of a sweet video posted by his ex-wife Demi Moore on Monday (AEDT) to mark his 68th birthday.

“Happy birthday, BW! So glad we could celebrate you today. Love you and love our family. Thank you to everyone for the love and warm wishes – we all feel them,” Moore wrote.

The video shows Willis with wife Emma Heming, as well as his three daughters with Moore – Willis’ daughters Rumer, Scout, and Tallulah – at his birthday party.

It shows the group singing Happy Birthday, with the Die Hard actor joining in for part of the song, chatting with his guests and blowing out birthday candles.

He can also be seen stumbling slightly more than once, including after making a big effort to blow out candles on a birthday pie.

Willis’s extended family revealed in February that he had been diagnosed with frontotemporal dementia.

It came just under a year after they revealed his immediate retirement from acting after being diagnosed with aphasia. It causes loss of the ability to understand or express speech.

The action star retired from acting immediately after his first diagnosis, and has kept a low profile since.

Moore’s birthday video came hours after Heming marked her husband’s big day with her own social media post.

“Today is my husband’s birthday. I have started the morning by crying. As you can see by my swollen eyes and snotty nose, I just think it’s important you see all sides of this,” she said.

Heming has been married to Willis since 2009. In her video, she said “people always tell me, ‘Oh you’re so strong. I don’t know how you do it’.

“I’m not given a choice. I wish I was but I’m also raising two kids in this,” she said – her daughters with Willis, Mabel and Evelyn.

“Sometimes in our lives, we have to put our big girl panties on and get to it, and that’s what I’m doing. But I do have times of sadness every day, grief every day and I’m really feeling it today on his birthday,” she said.

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It is the first update on Willis since Heming posted another video to her Instagram, pleading with paparazzi to give him “his space” after he was hounded for photographs during a rare public appearance with a group of friends for coffee in Santa Monica.

“Please don’t be yelling at my husband asking him how he’s doing or whatever — the woohoo-ing and the yippee ki yays … just don’t do it. OK?” she said.

“Give him his space. Allow for our family or whoever’s with him that day to be able to get him from point A to point B safely.”

FTD is often diagnosed at a younger age than other forms of dementia. Symptoms can include personality changes, speech difficulty and motor impairment.

Over a four-decade career, Willis’s movies had earned more than $US5 billion ($7.2 billion) at the worldwide box office.

He achieved initial fame for the 1980s comedy-drama TV series Moonlighting before he became the tough guy, action hero who shouted “Yippee Ki Yay” in five Die Hard films, and winning acclaim for roles in Pulp Fiction and The Sixth Sense.

In recent years, the prolific actor had primarily featured in direct-to-video thrillers.