For centuries, royalty and those less-than-royalty in Persia indulged in the traditional pastime of smoking opium.
The 1979 Islamic revolution forced such activity underground, but today its use - in the much harder, modern form of heroin - is making a comeback in Iran.
"Do you know what they are doing over there?" asks Rami, a former user, pointing his thumb at the crowd of 200 or so men dealing quietly among themselves in the shadow of Azadi monument, built to mark the 2,500th anniversary of the Persian Empire. "They're selling 'H.' "
Many of the men look poor, almost all are dressed in black and wearing beards, and there are some soldiers and police - even a woman or two, and a child - among them. Men playfully joust with each other; others are sprawled asleep on grass nearby. They are some of the 1 million addicts now estimated to be in Iran, despite vigorous efforts by the authorities to stamp out drugs.
"Everybody does it, from 15 years old to 50," says Rami, who has just finished a $100 rehabilitation course. But he is of two minds: He just bought another hit of heroin powder, but knows that if he uses it, progress could be undone, and he could be addicted once again.
Punishment is severe: Possession of 30 grams of heroin or 5 kilograms of opium is punishable by death.
Rami was first exposed to heroin years ago in northeast Iran, along the drug-smuggling border with Afghanistan. He was in the military and plied with illicit "gifts" of opium and heroin in exchange for favors.
Though the ruling clerics of the Islamic Republic have spent hundreds of millions combating the spread of drugs, making it one of their top priorities, security officers here appear not to intervene - even as pushers sell their illegal goods openly on a warm afternoon.
This leads Rami to his own conclusion: "I think the government wants this to happen," he asserts. "They don't want the youths to think."