Nasa leader Raila Odinga has been admitted at Mombasa Hospital after addressing rallies in Kilifi County, spokesperson Dennis Onyango has confirmed.
Mr Onyango, on his Facebook page, said that "we suspect food poisoning".
He said more information will be provided later.
Mombasa Governor Hassan Joho is at the hospital.
Mr Salim Lone has said the Nasa flag-bearer is responding well to treatment.
"Mr Odinga is feeling much better and the doctor just wants to observe him for the next one hour," he said.
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