Pakistan’s ousted prime minister, Imran Khan, was arrested on Tuesday in a major escalation of a political crisis that raises the prospect of mass unrest by his steadfast supporters. The crisis has been building for months as Mr. Khan has openly challenged the Pakistan military and the current government of conspiring against him. Mr. Khan, who was removed from office in a parliamentary no-confidence vote in April last year, is facing dozens of court cases on charges that include terrorism and corruption. The arrest instantly intensified a showdown between the current government and Mr. Khan, a populist former cricket star, who has staged a political comeback in the months since his removal from office. His party has drawn tens of thousands to political rallies across the country, at which Mr. Khan and others have called for fresh elections and accused Pakistan’s powerful military establishment of orchestrating his ouster.

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