Police and soldiers intercepted more than 850 young motorcyclists early Sunday and arrested 411 as the young people flocked to Vibhavadi Rangsit Road and a Saraburi petrol station for gatherings that alarmed motorists.
A member of the motorcyclist group in the Vibhavadi Rangsit area said they met early Sunday for a "charity trip", but the large size of the group worried motorists and police who were looking out for street racing, banned by the National Council for Peace and Order (NCPO).
About 30 police surrounded the riders who had filled up the express lanes near the Bang Khen intersection. They detained 227 men and 31 women. There were 59 teenagers, including some aged under 18 years old, including pillion riders.
Motorcycles modified for illegal racing will be seized under an order issued by NCPO chief and Prime Minister Prayut Chan-o-cha, under Section 44 of the interim charter.
"We don't seek to cause disturbances," said Wutthichai Phetkaeo, 27, whose group was travelling to Wang Takhrai waterfall in Nakhon Nayok.
Most riders in the group are from Nonthaburi and call themselves "Thurian Non Kan Yao", or "long stem Nonthaburi durian".
Mr Wutthichai said the trip to Nakhon Nayok was the group's second gathering after they travelled earlier this year to Bang Saen beach in Chon Buri to raise money to help the victims of the earthquake in Nepal on April 25. They raised 50,000 baht and sent it to Channel 3 TV which was overseeing the relief effort, Mr Wutthichai said.
But police detained members of the group saying some were riding in an unsafe manner and had modified their motorbikes for racing.
Wrongdoers aged over 18 will face charges in the Criminal Court, said assistant national police chief Pol Lt Gen Prawut Thawornsiri.
Several hundred motorcyclists and pillion riders sit on Vibhavadi Rangsit Road after police and soldiers stopped them on the way to what they said was a charity ride to Nakhon Nayok.
In Saraburi, 153 out of a group of more than 500 motorcyclists were also arrested and fined as they were found to be violating traffic laws, said Pol Lt Col Samran Noithung, deputy chief of Muang Saraburi police station.
Nipha Malai, 22, another member of the durian group, said she and her friends had met at the petrol station in Muang district for the trip to Nakhon Nayok to raise money for Wat Phra Phutthabhat Namphu in Lop Buri, known for its work with Aids patients.