Fear of instability has heightened in Zanzibar following a decision by the main opposition party, the Civic United Front (CUF), to boycott a rerun of last October's annulled election.
"CUF will not participate in a repeat election slated for March 20 because such an election is not legitimate," said Twaha Taslima, the CUF acting chairman.
He urged all Tanzanians "who love their country and believe in the rule of law, democracy and constitutionalism" not to participate in such an election.
He said the rerun was against the Zanzibar Elections Act (1984) and the Zanzibar Constitution.
The announcement, made on Thursday, means that CUF will neither be in the Isles government nor in the House of Representatives.
"The governing council urges all Zanzibaris to support their party and preserve peace knowing that preserving peace does not spell weakness but a way to encourage the international community to push for justice and democracy in Zanzibar," said Mr Taslima.
The party rejected the results of elections in Zanzibar in 2000 and 2005 on claims of rigging. It later closed ranks with the ruling CCM in 2010 to form a coalition government.
CUF had unilaterally declared victory in the October 25,, 2015 general election, before the Zanzibar Electoral Commission annulled the results over alleged irregularities. Local and international observers had deemed the polls free and fair.
Nassor Khamis and Ayub Bakari, who are ZEC members from the CUF, protested the annulment saying the seven-man commission had not been consulted over the decision as the law requires.
The Zanzibar Law Society (ZLS) said the decision to annul the results contradicted the earlier announcement in 31 constituencies.
There are 54 constituencies in Zanzibar, on Pemba and Unguja islands. CUF is dominant in Pemba, going by previous election results, and competes evenly with CCM in Unguja.
ZLS president Awadh Ali Said said the Zanzibar Constitution of 1984, Article 119(10) requires the ZEC to reach all its decisions by consultations. Six members -- two of whom must be the chairman and his deputy -- constitute a quorum, which Mr Said said was not met.
On January 22, ZEC boss Jecha Salim Jecha announced a rerun, which CUF had earlier said it would not participate in. The CUF governing council endorsed the decision in an emergency meeting on Thursday.
"There is no provision neither in the law nor the Constitution that empowers the commission chairman to annul an election and prepare a new one," Mr Taslima said.
The boycott means that CUF will lose the position of first vice president and several ministerial positions in the Government of National Unity.
The party said it was concerned about President John Magufuli's seeming indifference to the political impasse in Zanzibar, despite his having promised to resolve it when he addressed parliament for the first time last November.
CUF has asked the international community, human-rights watchdogs and the International Criminal Court to investigate what they call gross human-rights abuse, criminal acts and racism against the people of Zanzibar, and take action against the architects of such acts.
Political scientist Mwesiga Baregu said the decision "has distanced CUF from opportunistic politics" of the sort that places political power above the principles of democracy and constitutionalism.
"What is more important is to be sure whether the principles of democracy and rule of law have been adhered to; it is better to stand firm in defence of the law and the constitution," Mr Baregu said in an interview with The EastAfrican.
Zanzibar's 500,000 registered voters also cast ballots for Tanzania's Union president, and despite the cancellation on the islands, President Magufuli was sworn into office on November 5, 2015.
President Magufuli took office alongside East Africa's first female vice president, Samia Suluhu Hassan, who comes from Zanzibar.
CUF critics say the party supports Zanzibar's secession from the mainland and restoration of Arab ties -- claims which the party denies.
With the CUF boycott, CCM may face the easiest election in Zanzibar in recent years.
Hamad Rashid Mohammed, who contested on an Alliance for Democratic Change ticket, is likely to replace Hamad as first vice president, as he is likely to come second in the rerun, after CCM's Ali Mohammed Shein.
Under Zanzibar's Constitution, the winner in a presidential election takes the presidency, the runner-up takes the first vice presidency and the second vice presidency goes to someone from the president's party.
Mr Mohammed, a long-time critic of Mr Hamad inside CUF, left the opposition and join a new party, the ADC.
CCM in Zanzibar depends on the mainland for political support, especially in Zanzibar politics.
CCM sources say that January Makamba, the Minister of State in the Vice President's Office for Union Affairs and Environment and the man who managed the CCM presidential campaign, is likely to play a key role in helping CCM Zanzibar win the election. On Thursday, he declared the rerun legal.
However, it is not clear how Mr Makamba will bridge growing division within CCM Zanzibar considering the increasing numbers of dissenters.