The 40-year old Melo was detained for questioning at the central police station in the city from early afternoon, and was yet to be released on bail by the time we went to press last night.
When contacted by The Guardian, the Commander of the Dar es Salaam Special Police Zone, Simon Sirro, declined to comment on the matter, saying he would speak on Friday during his regular press briefing.
"Maxence was summoned by Dar es Salaam Zonal Crimes Officer, Camilius Wambura, this morning and he went straight to the central police station as directed," said Simon Mkina, a press freedom campaigner and one of JamiiForums' senior officials.
"He was accused of impeding police investigations into cybercrime allegations by not revealing the names and identities of some of JamiiForums' contributors," Mkina added.
JamiiForums is a Tanzania-based social networking website founded by Melo and other individuals in 2006. The online network runs user-generated content, and can boast of having more than 2.5 million visitors per month.
Mkina, who accompanied Melo to the police station, said a lawyer, Nakazael Tenga, tried unsuccessfully to secure his release yesterday.
"The police told me and the lawyer to leave the police station premises and said that Maxence will remain in their custody until tomorrow when he will appear before a Dar es Salaam court to face criminal charges," he said.
Insulting President Magufuli was made a criminal offense in Tanzania under a new Cybercrimes Act passed last year, punishable by up to three years in jail, a fine of around 7 million/-, or both.
A lecturer was in September charged with insulting the president in a WhatsApp message, bringing the number of people charged so far under the tough new law to 10.
The United States Millennium Challenge Corporation (MCC) in March canceled nearly $500 million of funding for Tanzania partly on concerns over enforcement of the new cybercrimes law.
The MCC said Tanzania has "engaged in a pattern of actions inconsistent with MCC's eligibility criteria", hence the decision to suspend its partnership.