LONDON — A British-Iranian woman held in a Tehran prison for years and her British husband began a joint hunger strike this week to demand her unconditional release, even as the relationship between the two nations has grown increasingly strained.

Nazanin Zaghari-Ratcliffe and her husband, Richard Ratcliffe, began their campaign on Saturday, and Mr. Ratcliffe set up a small campsite on the sidewalk outside the Iranian Embassy in central London. He remained there on Wednesday, despite embassy staff members erecting metal barricades between his tent and the front door, and intermittently sanding the railings behind him.

“Being here is meant to be a very visible presence to say, ‘This is unacceptable. You are holding an innocent person,’ ” he said in an interview on Tuesday evening. “They have effectively kidnapped my wife.”

He sat in a folding chair on the sidewalk in front of the embassy in a yellow waterproof jacket, slowly sipping bottled water as rain streamed from his cap onto his jeans, already soaked from a sudden downpour. He said he hadn’t eaten in four days.