TEHRAN (Reuters) - As night falls, rival supporters of Iran’s presidential candidates take over the streets of Tehran in boisterous scenes which at times resemble more a giant party than an election campaign in a conservative Islamic state.

A supporter of Iranian presidential candidate Mahmoud Ahmadinejad rides his motorcycle with the windshield covered with campaign posters during a rally in front of Tehran's Sharif University June 10, 2009. REUTERS/Ahmed Jadallah

In the run-up to Friday’s hotly contested vote, mainly young people descend on the capital’s most famous boulevard in the evenings and bring traffic to a standstill in a cacophony of chanting slogans, honking car horns and loud music.

But while many of those thronging tree-lined Vali-ye Asr in relatively affluent northern Tehran make clear their desire for political change, it also offers them a chance to let off steam and mingle with the opposite sex in public.

“For 80 percent ... they only come out to have fun,” said Ashkan, a teenager trying to make himself heard in the noise. He gave only his first name.

“It is an excuse for boys and girls to talk to each other without trouble,” he said, referring to the Islamic Republic’s ban on unrelated men and woman socializing.

As he spoke, swarms of motorbikes sped up the street, narrowly avoiding slow-moving cars and crowds of pedestrians. Some had passengers standing on the saddle cheering and waving the red, white and green Iranian flag.

A group of teenaged boys jumped out of their cars and started dancing to thumping Iranian pop music. Another group of young men and women shouted and waved pictures of their favored candidate as they walked in the middle of the dense traffic.

Such a public outpouring of emotion and jubilation has not been seen since Iran’s soccer team defeated the United States in 1998 or when reformist Mohammad Khatami swept to power by winning the previous year’s presidential election.

With more than 60 percent of Iran’s population under 30, the battle for the youth vote could be crucial for the outcome of an election pitting conservative President Mahmoud Ahmadinejad against moderates seeking political and social change.

AHMADINEJAD “NOT ALONE”

Police were deployed along Vali-ye Asr but they watched the carnival-like scenes without interfering. Despite reports of sporadic street clashes elsewhere in Tehran in recent days, there was no sign of tension or violence.

“Our young people don’t often get a chance to say what they want. They like to express themselves by coming to the streets,” said one middle-aged onlooker, Hamid Saeedi.

Northern Tehran, on a slope stretching toward the snow-capped Alborz mountains, is dominated by supporters of former Prime Minister Mirhossein Mousavi, a moderate who is seeking to deny Ahmadinejad a second four-year term.

Some supporters of Mousavi, who has attacked Ahmadinejad’s “extremist” foreign policy and says he would seek detente with the West, drove expensive four-wheel drives plastered with photographs of the bearded, bespectacled 67-year-old.

Others wore headbands and wristbands in his green campaign colors, leaning out of car windows and flashing victory signs.

Ahmadinejad, a self-styled champion of the poor who often rails against the West, draws support mainly from poor southern Tehran and rural areas where people have benefited from his handouts of cash and investment projects to help those in need.

But his backers make their presence felt also in this part of Tehran, trying to counter Mousavi’s camp at their home turf.

“I’m here so Ahmadinejad knows that he is not alone,” said one young, tall 22-year-old giving his first name as Mehdi. He sported a headband in the colors of the Iranian flag, which has become the symbol of the conservative president’s campaign.

A man who was selling green balloons to Mousavi supporters also said he would vote for the incumbent, who came to power in 2005 pledging to share out Iran’s oil wealth more fairly.

“This is purely business,” said Saeed Ebrahimi, 25, who came from a poor area. “Ahmadinejad is the only one who cares about our pain. He gave us water and electricity.”