Anyone with an allergy to horses would be wise to avoid Hyde Park this weekend, when about 100 riders and their mounts will gather for Horseman's Sunday, an annual celebration based on the Harvest Festival's sentiments of thanksgiving which includes a blessing of the animals by a vicar on horseback.

Ross Nye has organised the event since 1968. 'I started it off mainly to draw attention to the fact that there are a lot of horses around London,' he says. 'I thought we could have an event like the Horseman's Sunday which used to take place at Epsom racecourse.'

The turn-out fluctuates according to the weather, but the average is between 80 and 90 animals. 'The number of horses has never exceeded 112 but that's still a lot of animals. It sounded like the cavalry coming.'

There is no typical member of the congregation: they can be tall, short, fat, thin, hairy or grey. The horses come in all shape and sizes too: from a tiny champion Shetland pony ('You've got to look twice to see him') to chestnut thoroughbreds. 'We may even get a trotter or two with their old horsedrawns if they feel like turning up. I'm sure if I offered them a tenner they'd all come but we don't do that sort of thing.'

The owners park their horseboxes on the north side of Hyde Park and take the horses up Albion Street to Hyde Park Crescent. Despite having to stand still for the 30-minute service, the horses are generally well-behaved. 'We've had one or two who think they're circus horses, prancing and dancing all over the place, but so far there has not been any buckjumping or rodeos.'

Leading Sunday's service for the 12th time is Thaddeus Birchard from the Parish of St John, Hyde Park Crescent, who rides with embroidered vestments thrown over the back of his horse. 'He's quite a character,' Nye says. 'He's originally from New Orleans and he had never ridden a horse before he came here. It's a standing joke in the parish that when the churchwarden seeks out a new vicar, he asks them, 'Can you ride?'.'

One of this year's most interesting animals will be a Dale, from the Lake District. 'They're very short and stocky but they grow a prodigious amount of mane and the lady who's bringing him says his mane is nearly three feet long. He's going to attract a lot of oohs and aahs.'

Horse blessing: 11.30am Sun, St John's, Hyde Park Cres, W2. Show: 1.30pm Sun, Kensington Gardens (west end), W8

(Photograph omitted)