JAKARTA (REUTERS) - Indonesian presidential frontrunner Joko "Jokowi" Widodo has picked former vice president Jusuf Kalla as his running mate for July's election, a senior party official said on Monday.

A coalition of four parties led by Jokowi's Indonesian Democratic Party-Struggle (PDI-P) gathered in central Jakarta to make the announcement.

"After meeting all the party chairpersons...it was agreed on Jusuf Kalla," PDI-P vice secretary general Tjahjo Kumolo told reporters.

Recent opinion polls show a Jokowi-Kalla ticket is by far the most popular pairing for the July 9 election.

Candidates and their running mates must be registered by May 20.

If no candidate wins more than 50 per cent of the vote in first round, there will be a second round in September.

Kalla served as vice president during current leader Susilo Bambang Yudhoyono's first term in office from 2004 to 2009, where he gained the reputation of being a can-do politician.

More recently, the 72-year-old businessman from the east of the country has been head of the Indonesian Red Cross.