More than 600 family and friends are thought to have arrived so far.
Some 200 people packed into the church, which can seat 150, while more than 400 stood on the grass outside.
The Rev Emma Davies described Claire as a "truly beautiful" person who "sparkled".
– The Rev Emma Davies
We gather to remember Claire and to give great thanks to Claire who gave so much to so many people.
A woman whose supporters helped raise more than £1 million for charity after her death in this year's London Marathon will be buried next to her brother today.
Family and friends of Claire Squires will attend a private funeral service at St Andrew's Church in North Kilworth, Leicestershire, this afternoon after she collapsed and died less than a mile from the finishing line of the 26.2-mile course on Birdcage Walk near Buckingham Palace on April 22.
The 30-year-old was running for the Samaritans, in recognition of the work done by her mother Cilla, who had volunteered for the charity for 24 years.
The charity fund raising page for Claire Squires, who collapsed and died during the London Marathon last weekend has now reached £1m.
She was raising money for the Samaritans, who her mother Cilla had volunteered with for more than 20 years.
Miss Squires, 30, will be buried on Wednesday at St Andrew's Church in the village of North Kilworth, Leicestershire next to her brother Grant, who died in 2001 aged 25.
The donation website JustGiving has confirmed that £1,000,000 has been raised on the page of Claire Squires who collapsed and died during the London Marathon. When Gift Aid is added to the total, it pushes the level of donations past the million mark.
Miss Squires, 30, whose death resulted in an outpouring of generous donations in her name, will be buried at noon on Wednesday at St Andrew's Church in the village of North Kilworth, Leicestershire. She will be buried next to her brother Grant, who died in 2001, aged 25.
The coroner at the inquest into his death recorded an open verdict - it has been reported he was a drug user and was depressed after being involved in a car crash in which his girlfriend was killed.
Friend Nicola Short told the BBC: "I don't think any of us had any idea of how much she would raise. I think she would be completely overwhelmed and completely over the moon. She'd be going 'woo hoo', excited."
Donations are continuing to flood into Claire Squire's Just Giving Page. People from 60 countries have donated, and the money will be given to the Samaritans.
Volunteers there say the contributions mean some good will come from her death.
Initially Claire wanted to raise £500 but she is now set to become one of the biggest individual fund raisers in the history of the London Marathon