The 5km run, which is free to enter and open to people of all ages and abilities, will now take place in the park at 9am each Saturday throughout the year.
Cassiobury parkrun is a timed run, not a race, and is coordinated entirely by local volunteers. It aims to break down barriers to taking part in regular exercise in a fun and welcoming environment. Participants receive their time by text and email after the run, and they can track their improvement over time on the parkrun website.
The event was funded by Watford Borough Council and is completely free to enter. Participants simply register on the parkrun website and print out their unique barcode. The barcode is then valid at any of the 500 park runs held worldwide every Saturday, which attract around 100,000 participants each week.
Mayor Dorothy Hill, who cut the tape and sent the 157 runners on their way, said: "Cassiobury parkrun is great fun, an amazing way to keep fit - which makes you feel wonderful, and it's suitable for people of all fitness abilities. The volunteers who manage parkrun are so supportive, and where better to take part in the event than in the beautiful and historic surroundings of Cassiobury Park."
Andy Nice, Cassiobury parkrun Event Director said: "We are delighted to quite literally get up and running after many months of planning. The feedback from the runners and volunteers was fantastic and it was great to see so many people of all ages, including families, running together around this beautiful park. We even had one person who regularly runs Gdansk parkrun in Poland, who was visiting London on the weekend so came along to run with us."
Glen Turner, Regional Ambassador for parkrun UK, said: "This great new event has capped a wonderful week for parkrun during which we registered our one millionth UK participant. We have been working hard to bring parkrun to the Watford area for a long time now, so it's been great to welcome both South Oxhey and Cassiobury to the family over the past four weeks.
"We always say that parkrun is open to everyone, from beginners to Olympians, and Saturday was no exception. It was great to see Team GB marathon runner Claire Hallissey, who competed in the London Olympics, running alongside people who were taking part in their very first 5k event."
For the record, Kerr Miller was the first man home in 17 minutes 22 seconds, with Claire Hallissey leading the women in 18:03. Charlie Scott was the fastest junior male in 21:53, while Violet McMullan was the first junior girl in 27:36.
Registration is free at www.parkrun.org.uk/cassiobury or email firstname.lastname@example.org for information.