The former president was only being taken for a check-up, said a top official of the Rapid Action Battalion (RAB).
Speculation was rife that Ershad, whose Jatiya Party is a key ally of the Awami League-led grand alliance, was "detained" as part of the government's efforts to force him to participate in the January 5 polls.
However, RAB media wing Director Habibur Rahman told Bdnews24 that Ershad was at home when he fell sick. "The personnel guarding his residence took him to the hospital after they were told of his feeling sick."
Activists of Jatiya Party scuffled with security personnel who took Ershad to the Combined Military Hospital where he was admitted.
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TV footages showed a grim-faced Ershad was sitting on the backseat of the SUV wearing a red sweater as the convoy of police and RAB vans drove off.
"It is mysterious, I was with him (Ershad) until the evening...He was in good health and also took part in his routine physical exercise," Ershad's press secretary Sunil Subho Roy told reporters.
Ershad's dramatic shift compounded the problems the interim government led by Prime Minister Sheikh Hasina at a time when the main opposition Bangladesh Nationalist Party has also announced it will boycott the polls.
His hospitalisation at the secured facility came amid a desperate effort by the Awami League to get the Jatiya Party, wracked with infighting and factional clashes, back to polls if required under the leadership of his wife Raushan,currently a minister in the poll time multi-party cabinet.
On December 5, Ershad had threatened to kill himself after security forces surrounded his residence. "I have loaded four pistols...I told the government I will kill myself if they play any tricks with me. I will die before the RAB or the police can lay a finger on me," he had said.