Parks have been closed or are being patrolled as police forces gear up for Halloween-related problems.
Shops in many areas of Wales have also been advised not to sell flour and eggs to under-18s.
In Cardiff, Grange Gardens in Grangetown is closed after reports of youths throwing fireworks and stones.
There have also been patrols at Penygraig Park and Belle Vue Park in the Rhondda after reports of anti-social behaviour.
Wales' four forces have warned youths about intimidating vulnerable residents, with Gwent Police hoping to avoid a repeat of last year.
Halloween proved to be one of its busiest nights of the year - with 607 calls mainly relating to crime, public safety and anti-social behaviour.
There have already been problems in Newport, with vandals smashing windows and damaging buses in areas including Ringland, by throwing missiles at them.
Bus operator NAT Group managing director Adam Keen said: "There does seem to be a certain demographic responsible for the recent attacks, and I think the combination of Halloween and half-term has exacerbated the issue."
Gwent Police's head of local policing for Newport, Chief Inspector Ian Roberts, said these were "serious crimes" that "endanger lives and disrupt essential community services".
Remember this #Halloween that not everyone sees the fun!— Gwent Police (@gwentpolice) October 30, 2019
Eggs have many uses; throwing them isn’t one!
We will be treating this as criminal damage and those caught will be prosecuted and could face a criminal record. pic.twitter.com/GxnbZsISsV
Dyfed-Powys Police said most communities it covered did not experience a great increase in crime, but it has taken some measures.
These include working with shop owners to make sure eggs and flour are not sold to anyone under 18 in the three weeks around Halloween.
"We don't want people's enjoyment to get out of hand and cause people to feel threatened in their own home," said Ch Insp Nicky Carter.
And while the noise of Bonfire Night can scare animals, RSPCA Cymru has said Halloween could actually benefit wildlife.
It is encouraging those who have carved pumpkins not to simply throw them in the bin when they are finished with them.
"Squirrels, foxes, badgers and birds all enjoy them," a spokesman said.
"So people could leave chopped up pumpkin outside in dishes for wild animals to eat if they choose.
Pigs and chickens can also eat pumpkins - but people were warned to make sure they had been cleared of candles.