Anti-bloodsports campaigners in Wales have warned they will continue to police hunts.
The David Davies Hunt is celebrating its centenary
The Wales Alliance against Cruel Sports said it would gather evidence and ensure that hunters breaking the new laws were prosecuted.
Thousands of people turned out to support hunts in Wales on Saturday, just hours after the ban on hunting with dogs came into force.
But Welsh police forces reported that all were operating within the law.
Around 100 Welsh hunts their followers turned out to mark the first day of the new ban a dramatic show of defiance.
There were foxes killed, but the Country Land and Business Association (CLA) said its members would take part in meetings as normal and had vowed to stay within the law.
Some anti-hunt campaigners turned up to watch meetings taking place, and the Wales Alliance Against Cruel Sports warned that its members would continue to gather evidence to pass on to the police.
"I believe that there are hunts that will break the law in Wales," said Alliance chairman Ralph Cook, who has spent 27 years campaigning against hunting for sport.
"We are going to have to get monitors there on occasions, and we will be doing that from November."
Hunts like the Llangeinor had many supporters out
Wiltshire police said four men - three of them from Wales - had been arrested under the new law near Malmesbury. They were alleged to have had four dogs and the remains of a hare. Hunt supporters have said they will continue efforts to overturn the ban.
The new legislation has been criticised by Montgomeryshire MP Lembit Opik, leader of the Liberal Democrats in Wales and co-chairman of the Middle Way Group.
He visited two hunts in his constituency on Saturday - the Tanatside Hunt in Welshpool, and the David Davies Hunt in Llandinam, near Newtown - and said he was "overwhelmed" at the show of support.
"There was a bigger turn out than I have ever seen in the past," he said. "There were about 500 people at the Tanatside and about 250 at the David Davies Hunt. "Their mood was very clear - sombre and determined, and actually quite optimistic that the ban will eventually be overturned."
Mr Opik - who said he had never hunted - said the hunts in his area were being "very careful to stay within the law".
At Hayscastle in north Pembrokeshire, about 80 riders turned out in full hunting regalia with a pack of hounds.
Hunt master Wyn Morris said the meet was intended as a show of defiance and to exercise the dogs.
They would not be hunting, he insisted, and if the hounds picked up a fox's scent, they would be called off. But he admitted that might be "pretty difficult".
Around 60 riders were seen off by several hundred supporters when the Flint and Denbigh Hunt met at Dolwen near Abergele.
There was no apparent police or anti-hunt presence. Huntsman Jeremy Reed said they intended to "exercise" the hounds in the morning and then use two of the dogs to flush a fox out to be shot. Both activities, he said, would be within the law.
None of the Welsh police forces reported hunting-related incidents. South Wales Police, however, said there had been several calls from members of the public saying they had seen hunts assembling.
Inspector Gareth Hawkins said staff were informing people that, although hunters were out, dogs were following scent trails, not hunting live animals. The League Against Cruel Sports said it had no evidence of illegal hunting in Wales.