Police have been forced to lob tear gas canisters at a section of the crowd attending the National Super Alliance (NASA) rally at Uhuru Park, Nairobi.
The Opposition supporters were reportedly protesting the presence of thieves who they claimed were harassing them during the event.
This caused a slight stampede as hundreds of the attendees scampered for safety.
NASA principals are set to announce the Alliance's presidential flag bearer and his running mate who will take on President Uhuru Kenyatta in the August 8 General Election.
Last week, the Opposition noted that it had finally selected the candidate having kept the country eager to know who would lead them in the upcoming polls.
The flag bearer is set to be selected from among four of the principals including ODM's Raila Odinga, Kalonzo Musyoka (Wiper), Musalia Mudavadi (ANC) and Ford Kenya's Moses Wetangula.
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