Iranian reformist presidential candidate Mehdi Karroubi has rejected calls by some reformists to pull out of Friday's election to avoid splitting the country's moderate vote.

Mr. Karroubi told a news conference in Tehran Tuesday he will remain in the race, despite his status as an outsider among the four candidates.

Some reformists want him to make way for their other candidate, Mir Hossein Mousavi, who is seen as the main challenger to the conservative incumbent, President Mahmoud Ahmadinejad.

Tens of thousands of Iranians joined competing rallies for Mr. Mousavi and Mr. Ahmadinejad in Tehran late Monday.

Mr. Mousavi's supporters say they formed a 19-kilometer long human chain running the entire length of the capital's main north-south avenue (Vali Asr).

Mr. Ahmadinejad's supporters held a mass rally in a central Tehran mosque. His aides say he left the venue without making a speech because he could not make his way through the crowd.

Some members of the two camps faced off on Tehran's streets, shouting slogans and waving flags. No violence was reported.

An opinion poll published Monday by two U.S.-based research institutes showed 34-percent of Iranian respondents backed Mr. Ahmadinejad, while 14 percent picked Mr. Mousavi and 27 percent were undecided.

The fourth candidate in the race is a conservative outsider, Mohsen Rezaei.

If no candidate gets at least 50 percent of the vote, the top two vote-winners will compete in a June 19th run-off election.

Mr. Mousavi appears to have gained support in recent weeks among younger voters and city dwellers who use the Internet to mobilize rallies.  Mr. Ahmadinejad is popular among rural voters and has the backing of the Basij religious paramilitary group with millions of members.

Reformists blame Mr. Ahmadinejad for Iran's high inflation and unemployment. The Iranian president says the economy has improved under his leadership.

Some information for this report was provided by AP and Reuters.