MANDALAY, Myanmar (AP) — About a thousand demonstrators against last month’s military seizure of power in Myanmar have ventured onto the streets of the country’s second-biggest city, those in the vanguard carrying homemade shields bearing images of the three-fingered salute, the movement’s symbol of defiance. Tuesday’s marchers in Mandalay gathered for just a few minutes before dispersing to avoid confronting riot police. In Yangon, the biggest city, a vocational school principal involved in the peaceful resistance movement has died of unknown causes after being taken into custody by security forces, said reports in the country’s media and an activist who knew him. The unexplained death of Zaw Myat Lin was the second to occur in such circumstances in recent days.