Harare - Zimbabwe's opposition is talking up an anti-Robert Mugabe coalition for the 2018 elections but differences over strategies and implementation are threatening the establishment of such an electoral alliance.The southern African country's former prime minister Morgan Tsvangirai has called on opposition leaders to gang up against President Robert Mugabe in the 2018 elections but warned that there must be trust among the country's opposition leaders if the coalition was to be successful.This comes at a time when Zimbabwe's main opposition Movement for Democratic Change (MDC) founding president snubbed coalition talks held in Cape Town, South Africa, this week.The talks were attended by leaders of thirteen other smaller parties. The meeting was organised by a South African think-tank, In Transformative Initiative, with a view to finding a single presidential candidate that would challenge Mugabe in 2018. Pro-democracy forces "I did not see it necessary to go to that meeting; have we failed as Zimbabweans to organise ourselves in Zimbabwe? But that doesn't underplay the issue of a coalition. How do you introduce a subject that you have not planned with me?" said Tsvangirai. Tsvangirai broke ranks with his lieutenants, Welshman Ncube, Tendai Biti and Elton Mangoma who went on to form their own parties. But most Zimbabweans and the opposition parties concur that there is need to establish a coalition of all pro-democracy forces in order to unseat Mugabe who has been in power since the country's independence."There is a national sentiment that we must unite and we will unite as we move on but it's a process not an event and I hope that everyone will understand that it's a question of building trust among the political leadership so that the people will trust us. But you do not leave a party and you go out there and say let's hold coalition talks."Meanwhile, the parties that met for talks in South Africa released a statement on Friday saying they were committed to do "all that is necessary" to form a loose election coalition ahead of the 2018 polls. "We will meet by the end of the year (2016), to finalise the modalities of our new working relationship. We also commit to reach out to other leaders from different sectors in our nation," reads part of the communique.