"After tests, doctors started treating him for fatigue. The doctors have further recommended complete bed rest for a few days and they will confine him in hospital for that period," said a statement from his spokesman, Dennis Onyango.

Together with President Mwai Kibaki, Odinga leads the grand coalition government that was formed in 2008 to end weeks of bloodshed that followed a disputed presidential election. Both leaders are backing the new constitution.

Voters go to a referendum on August 4 to decide whether to adopt the proposed new constitution, which is seen as important to securing investor confidence in the region's largest economy.

Opposition to the draft is led by Higher Education Minister William Ruto, several lawmakers and church leaders who are unhappy over two clauses in the draft that will enshrine Muslim Kadhi courts in the constitution and legalise abortion under certain circumstances.