Iran’s Elnaz Rekabi, who competed without hijab, in Tehran

Iranian competitive climber Elnaz Rekabi received a hero's welcome on her return to Tehran early Wednesday, after competing in South Korea without wearing a mandatory headscarf required of female athletes from the Islamic Republic.

Rekabi's decision not to wear the hijab while competing Sunday comes as protests sparked by the Sept. 16 death in custody of a 22-year-old woman have entered a fifth week. Mahsa Amini was detained by the country's morality police over her clothing, and her death has seen women removing their mandatory hijabs in public.

The demonstrations, drawing school-age children, oil workers and others to the street in over 100 cities, represent the most-serious challenge to Iran's theocracy since the mass protests surrounding its disputed 2009 presidential election.

Supporters and Farsi-language media outside of Iran have worried about Rekabi's safety after she choose to compete without the hijab.

An Instagram post Tuesday on an account attributed to Rekabi described her not wearing a hijab as "unintentional," though it wasn't immediately clear whether she wrote the post or what condition she was in at the time. The Iranian government routinely pressures activists at home and abroad, often airing what rights group describe as coerced confessions on state television.

Video shared online showed large crowds gathered early Wednesday at Imam Khomeini International Airport outside of Tehran, the sanctioned nation's main gateway out of the country. The videos, corresponding to known features of the airport, showed crowds chanting the 33-year-old Rekabi's name and calling her a hero.

She walked into one of the airport's terminals, filmed by state television cameras and wearing a black baseball cap and a black...