Family and friends of the man regarded as the driving force of Scottish theatre turned out in bright-coloured clothes to honour Mr MacLennan at the memorial in Clydebank.
He died last Friday after revealing earlier this year that he had been diagnosed with terminal motor neurone disease. He was 65.
A piper played as mourners including playwright and artist John Byrne and actors Dave Anderson and Colin McCredie entered Clydebank Crematorium.
Still Game stars Greg Hemphill and Ford Kiernan also turned out to pay their respects.
Mr MacLennan’s son Shane read a poem written by his father during the service and everyone joined in with a performance of the Robert Burns song A Man’s A Man For A’ That.
His brother Robert told people packed into the hall, and those spilling out into the reception area: “His life has enriched us all.”
There was laughter and clapping as friends paid tribute to Mr MacLennan, who helped establish the influential 7:84 and Wildcat theatre companies in the 1970s.
He is also hailed as the man behind the popular lunchtime theatre programme A Play, A Pie And A Pint which began in Glasgow and has since been reproduced internationally.
Since it began in 2004, thousands of people have enjoyed the work of new and established writers at the Oran Mor theatre in the city’s west end, where people were invited to gather following the service.
Mr MacLennan was pictured on the back of the order of service enjoying one of his favourite pastimes, fishing. He is survived by his wife, the actress Juliet Cadzow.
A collection was taken at the service for the MND (Motor Neurone Disease) Association.