Saraswati was arrested as he refused to end his fast despite his deteriorating health, Haridwar District Magistrate Sachin Kurve said.
"We had taken medical opinion on his health and doctors had advised him to end his fast immediately," he said.
Saraswati has been taken to a local court and further action will depend on its directions, he said.
The activist had been on a fast unto death since December 10 at Haridwar's Matrisadan Ashram to protest resumption of quarrying and stone crushing in the ecologically sensitive areas along the river in alleged violation of the Ministry of Environment and Forest norms.
"As he declined to break his fast till the government withdraws its December 1 order allowing resumption of these activities despite repeated pleas by us, we had no choice but to arrest him," Kurve said.
A team of officials led by Haridwar SDM Arvind Pandey Singh had made an infructuous attempt to persuade Sarawati to end his fast yesterday.
Saraswati had shut himself in a room on Thursday to foil any attempt by officials to forcibly shift him to a hospital.
