The 16-year-old was reported missing on 19 February. Body parts were found at a house in the Barton Hill area of Bristol on 2 March.
Organisers said although the walk "won't bring Becky back" it will help "give this beautiful young girl the send-off she deserves".
Becky's stepbrother Nathan Matthews, 28, has been charged with her murder.
Friends have sold wristbands, T-shirts and balloons to help support her family.
The walk started at 11:00 GMT on The Centre and is making its way to Castle Park where balloons are due to be released in her memory.
Earlier this week, Mr Matthews, from Warmley, Bristol, appeared by video link at Bristol Crown Court.
He has yet to enter a plea to the charge and was told a provisional trial date had been set for October.
His girlfriend, Shauna Hoare, 21, who is charged with perverting the course of justice, also appeared by video link.
Four other defendants have been charged with assisting an offender.

Bowe, 30, scored the second of Ireland's tries in Saturday's 29-15 win over South Africa at the Aviva Stadium.
He has won 55 caps for Ireland, scoring 27 tries, and has made 124 appearances for Ulster, scoring 48 tries.
Earlier this week, the IRFU confirmed that Rob Kearney had also signed a new three-year deal to stay with Leinster.
Bowe made his competitive debut for Ulster against Connacht in April 2004 and enjoyed a spell with the Ospreys, before returning to his home province for the 2012-13 season.
He made his Ireland debut against the USA in November 2004 and has twice toured with the British and Irish Lions.
The Monaghan man played in all three Lions Tests against South Africa in 2009, and in two Tests in Australia in 2013.
He was a member of Ireland's 2009 Grand Slam-winning team and was named Six Nations Player of the Championship in 2010.
"I'm delighted to have signed a new contract with the IRFU and Ulster," Bowe said.
"It is an exciting time for both the province and the national team and I hope to play a part in achieving success on both fronts in the years to come."

Three Pugs, a Chihuahua, a Jug - a Pug crossed with a Jack Russell - and a Puggle, a Pug-Beagle crossbreed, were taken in Kemble Road, Forest Hill.
A witness said they saw two men load the animals into a black VW van and drive off.
The Met confirmed four dogs were swiped in a similar incident outside a house in nearby Streatham in July.
Dog owner Yuriko Matsukawa, whose five-year-old pug Pia was taken earlier, said the stolen pets were valuable, with pug puppies worth around Â£1,000.
She added: "I'm devastated. I'm desperate to be reunited with her."
The Evening Standard reported the dogs in Streatham were stolen from a dog walker's van in Glennie Road.
The Met Police confirmed the pets were still missing.
The Greater London Authority said last month there had been an an increase in dog thefts in the capital since 2013.
It found dog thefts made up the majority of domestic pet thefts across London and had increased from 186 in 2014 to 217 in 2015.
Hundreds of people are taking part in a fundraising walk in Bristol to remember Becky Watts who was killed last month.
[NEXT_CONCEPT]
Ireland winger Tommy Bowe will remain with Ulster until at least June 2018 after signing a new three-year contract with the IRFU.
[NEXT_CONCEPT]
Six pets worth thousands of pounds have been stolen from a dog walker's van in south London.