Thousands of Thais took to the streets of the capital on Sunday demanding reforms of the monarchy, defying a court ruling that such demands are a veiled attempt to overthrow the institution.

Youth-led protests that began last year by calling for the removal of Prime Minister Prayuth Chan-ocha, 66, a former coup leader, have become the biggest challenge in decades to the monarchy which is constitutionally enshrined to be held in "revered worship"...