PARIS — Iran's top police officer issued an ultimatum on Monday to protesters who joined what authorities have deemed "riots", saying they must hand themselves in within three days or face the full force of the law. But the government also pledged to tackle economic hardships that sparked the demonstrations, which were met with a crackdown that rights groups say has left thousands dead. The protests constituted the biggest challenge to the Iranian leadership in years, with the full scale of the violence yet to emerge amid an internet blackout. National police chief Ahmad-Reza Radan on Monday urged young people "deceived" into joining the "riots" to turn themselves in and receive lighter punishment. Those "who became unwittingly involved in the riots are considered to be deceived individuals, not enemy soldiers" and "will be treated with leniency", he told state television. Officials have said the demonstrations were peaceful before descending into chaos fuelled by Iran's arch-foes the United States and Israel in an effort to destabilise the nation. The heads of the country's executive, l