Kenyatta, 55, is seeking a second and final term after winning the 2013 race against veteran politician Raila Odinga, 72, whose Orange Democratic Movement is one of the five main opposition parties uniting.

The test will be whether the new coalition can agree on a single candidate before the vote without splintering.
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Kenyatta's presidency has seen the economy grow at a steady 5 to 6 percent a year, but has faced headwinds from a spate of Islamist militant attacks that have killed hundreds of people. He has faced criticism for not doing more to tackle corruption.

Some opinion polls have suggested Kenyatta is ahead, although it is yet to be confirmed who his main rival will be. However, voting in Kenya in the past has tended to be driven by tribal loyalties rather than policy.

Odinga, who has lost three elections, wants to run again but is facing challenges from others in the opposition alliance. He disputed the results of the last vote in 2013 and the election in 2007, which was followed by weeks of ethnic violence in which about 1,200 people died.

The build-up to this year's vote has already been marred by clashes between protesters and police, sparked by a row over who sits on a committee overseeing elections. At least four people died in the violence.

The government agreed to change the commissioners, whom the opposition accused of bias. New members are now being appointed.

At Wednesday's launch rally for the new National Super Alliance of Kenya (Nasa), opposition politicians lined up in a packed Nairobi cultural center to call for unity.

"Pray for us to solidify, to be one, to be united, and we shall win," said Musalia Mudavadi, whose Amani National Congress Party is a member of the alliance.

The meeting also aimed to agree a response to an election law amendment, pushed through by the government, which the opposition said could lead to rigging by allowing the use of manual systems if electronic voting systems failed.

Electronic systems that were meant to ensure a transparent vote in 2013 collapsed, which the opposition said undermined the legitimacy of the vote.

Some opposition politicians have previously called for protests against the election law amendment. But participants at Wednesday's rally played down this option.

"We would have gone to the streets. But we have chosen to go register as voters," said Nick Salat, secretary general of KANU, another of the groups in the new alliance which also includes the Wiper Party and Ford Kenya Party.

(Writing by Edmund Blair; Editing by Mark Trevelyan)