Beginning in April 1949, there was considerable pressure on Nkrumah from his supporters to leave the UGCC and form his own party. On 12 June 1949, he announced the formation of the Convention People's Party (CPP), with the word "convention" chosen, according to Nkrumah, "to carry the masses with us." There were attempts to heal the breach with the UGCC; at one July meeting, it was agreed to reinstate Nkrumah as secretary and disband the CPP. But Nkrumah's supporters would not have it, and persuaded him to refuse the offer and remain at their head.  The CPP adopted the red cockerel as its symbol - a familiar icon for local ethnic groups, and a symbol of leadership, alertness, and masculinity. Party symbols and colours (red, white, and green) appeared on clothing, flags, vehicles, and houses. CPP operatives drove red-white-and-green vans across the country, playing music and rallying public support for the party and especially for Nkrumah. These efforts were wildly successful, especially because previous political efforts in the Gold Coast had focused exclusively on the urban intelligentsia.  The British convened a selected commission of middle-class Africans, including all of the Big Six except Nkrumah, to draft a new constitution that would give Ghana more self-government. Nkrumah saw, even before the commission reported, that its recommendations would fall short of full dominion status, and began to organise a Positive Action campaign. Nkrumah demanded a constituent assembly to write a constitution. When the governor, Charles Arden-Clarke, would not commit to this, Nkrumah called for Positive Action, with the unions beginning a general strike to begin on 8 January 1950. The strike quickly led to violence, and Nkrumah and other CPP leaders were arrested on 22 January, and the Evening News was banned. Nkrumah was sentenced to a total of three years in prison, and he was incarcerated with common criminals in Accra's Fort James.  Nkrumah's assistant, Komla Agbeli Gbedemah, ran the CPP in his absence; the imprisoned leader was able to influence events through smuggled notes written on toilet paper. The British prepared for an election for the Gold Coast under their new constitution, and Nkrumah insisted that the CPP contest all seats. The situation had become calmer once Nkrumah was arrested, and the CPP and the British worked together to prepare electoral rolls. Nkrumah stood, from prison, for a directly-elected Accra seat. Gbedemah worked to set up a nationwide campaign organisation, using vans with loudspeakers to blare the party's message. The UGCC failed to set up a nationwide structure, and proved unable to take advantage of the fact that many of its opponents were in prison.  In the February 1951 legislative election, the first general election to be held under universal franchise in colonial Africa, the CPP was elected in a landslide. The CPP secured 34 of the 38 seats contested on a party basis, with Nkrumah elected for his Accra constituency. The UGCC won three seats, and one was taken by an independent. Arden-Clarke saw that the only alternative to Nkrumah's freedom was the end of the constitutional experiment. Nkrumah was released from prison on 12 February, receiving a rapturous reception from his followers. The following day, Arden-Clarke sent for him and asked him to form a government.

Answer this question "when did this start" by extracting the answer from the text above.
Beginning in April 1949, there was considerable pressure on Nkrumah from his supporters to leave