Adelaide's Christmas Pageant has drawn an estimated 345,000 people, with organisers claiming it as one of the biggest crowds in years.

Numbers were up by about 20,000 on last year's event as hundreds of thousands of people lined King William Street and North Terrace to watch more than 60 floats and hundreds of performers.

The event, which marks the countdown to Christmas, is in its 82nd year.

Pageant director Brian Gilbertson said it was always an amazing spectacle and this year was no different.

"We've got counters out there and we wait till they come in, also the police do an assessment and we have a look at the two, but it is a big crowd," he said.

"We walked the whole pageant route this morning and I don't think I've seen it this big for a while."

Many hands go into the preparation of the pageant, which takes more than a year to organise.

"We start planning for any pageant the September of the year before, so it takes a while to get it together," Mr Gilbertson said.

"And then between then and the pageant, nearly 4,000 people are involved. When you stop and think about the logistics side of it with the police, emergency services, St John's, SA Ambulance, all of those groups who assist us behind the scenes."

The city's streets began filling up as early as 3:00am (ACDT) with parents lining up to get the best spot for their children.

"About 3:15am I walked along there and every metre each side of the road was covered - at that time of the morning! It's amazing what people do for their kids," one dad said.

Adelaide resident Karen Gabriel came with her grandchildren and said it has always been a huge day.

"We've been here since 8:30am. I just think it is great for all the kids, I love to see their reaction, it's just wonderful, absolutely," she said.