Tanzanians largely ignored on Thursday a US-based activist's call to stage nationwide protests against President John Magufuli's government.

Nine demonstrators marched in Tanzania’s commercial capital, Dar es Salaam, on Thursday. They were quickly arrested.

Mange Kimambi, a self-styled Tanzanian democracy activist based in the United States, tried to organise the demonstrations through social media to coincide with Thursday’s anniversary of the union between mainland Tanzania and the Indian Ocean archipelago of Zanzibar.   

"As Tanzanians, we must defend our peace. We should not allow ourselves to be used by our enemies," Magufuli said in his Union Day speech to the nation, broadcast live on state television.    

"I will defend our union at all costs. We will not have mercy on anyone - inside or outside the country - who tries to destabilise us."   

Some shops were closed in several towns, but most of the country was peaceful, with no reports of street protests, according to witnesses.

Heavily armed police have patrolled major towns and cities across the country over the past three days. One senior police officer warned on Wednesday that protesters would be "beaten like stray dogs".

The main opposition party, CHADEMA, said several of its leaders were arrested by the police in various parts of the country on allegations of inciting Tanzanians to take to the streets.   

Police in Dar es Salaam on Thursday confirmed the arrest of a leader of CHADEMA's women's organisation for mobilising Tanzanians to march against the government.   

Tanzania's JamiiForums online platform said on Thursday it suffered from a cyberattack designed to disable it, forcing it offline temporarily.   

"We are back," JamiiForums tweeted. "We were offline due to a distributed denial of service attack. We apologise for the inconvenience this attack has caused and we are currently reviewing the incident to ensure any learning points are used to enhance our protection in future.

JamiiForums did not say who was behind the attack, but its co-owners are facing criminal charges over a user privacy battle with the government. Authorities had accused website administrators of obstructing a police investigation by refusing to disclose the identity of some of its users.