Amnesty International has called on authorities to free a group of 20 activists, mostly students, arrested for political gatherings and distributing "inappropriate reading material" to people last week.
The organisation posted a message on its website, demanding authorities release the protesters.
Amnesty International Senior Research Adviser for Southeast Asia and the Pacific, Champa Patel, wrote on the website: "These crude tactics represent the latest in series of attempts by Thai military authorities to muzzle dissent," Ms Patel said.
"If a small group activists cannot hand out leaflets, then what hope is there that the rights to freedoms of expression and assembly will be respected in the run-up to the referendum?
"If the military authorities wish to recover some of their much-demanded credibility on human rights, they must stop cracking down on peaceful activists and drop all charges against them."
On June 23, a group of 13 people led by pro-democracy activist Rangsiman Rome, who is also a member of the "New Democracy Movement", were arrested for political gatherings and for running a "vote no" campaign for the referendum on the draft constitution.
Campaigners wearing "vote no" campaign T-shirts had distributed leaflets about the Aug 7 referendum on the draft charter to people in Keha Bang Phli market in Bang Sao Thong district, Samut Prakan.
Before they started their activity, a group of military officers and local officers approached the campaigners, and asked to check the documents they intended to hand out to locals in the market.
Authorities allowed them to hand over leaflets on the referendum containing information provided by the Election Commission.
But the group defied their orders and distributed leaflets they had prepared themselves to promote a "vote no" stance on the referendum.