by Sheikh Sabiha Alam and Mohammad Mazed

Bangladesh's former prime minister Khaleda Zia, who many believed would sweep elections next year to lead her country once again, died on Tuesday aged 80.

The government declared three days of state mourning for the country's first woman prime minister, with vast crowds expected to attend her funeral on Wednesday.

Despite years of ill health and imprisonment, Zia vowed in November to campaign in elections set for February -- the first vote since a mass uprising toppled her arch-rival Sheikh Hasina last year.

Zia's Bangladesh Nationalist Party (BNP) is widely seen as a frontrunner, and her son Tarique Rahman, who returned only on Thursday after 17 years in exile, is seen a potential prime minister if they win a majority.

"The country mourns the loss of a guiding presence that shaped its democratic aspirations," Rahman said in a statement.

He said he was also mourning the loss of the "infinite love" of his mother, who "endured repeated arrests, denial of medical care, and relentless persecution".

"Yet even in pain, confinement, and uncertainty, she never stopped sheltering her family with courage and compassion. Her resilience... was unbreakable."

In late November Zia was rushed to hospital, where, despite the best efforts of medics, her condition deteriorated from a raft of health issues.

Nevertheless, hours before her death, party workers had on Monday submitted nomination papers on her behalf for three constituencies for the polls. The BNP said Zia died shortly after dawn on Tuesday.

Interim leader Muhammad Yunus said Bangladesh "has lost a great guardian".

"Through her uncompromising leadership, the nation was repeatedly freed from undemocratic conditions and inspired to regain liberty," Nobel Peace Prize winner Yunus said in a statement.

Indian Prime Minister Narendra Modi said he hoped Zia's "vision and legacy will continue to guide our partnership", a warm message despite the strained relations between New Delhi and Dhaka since Hasina's fall.

Pakistan's Prime Minister Shehbaz Sharif said Zia had been a "committed friend" to Islamabad, while China's ambassador in Dhaka Yao Wen offered his condolences.

"China will continue to maintain its longstanding and friendly ties with the BNP," he said.

'Prison over luxury'

Braving cold rain, mourners gathered on Tuesday outside the hospital in Dhaka where Zia's body rests.

"This is an irreparable loss for the nation," senior BNP leader Ruhul Kabir Rizvi told reporters, his voice choking with emotion.

"She chose prison over luxury and spent years behind bars," said Golam Kibria, 29, a BNP loyalist who said he was tortured under Hasina's government, calling Zia an "unmatched leader who can never be replaced".

Three-time prime minister Zia was jailed for corruption in 2018 under Hasina's government, which also blocked her from travelling abroad for medical treatment.

Zia was released last year, shortly after Hasina was forced from power.

Hasina, 78, sentenced to death in absentia in November for crimes against humanity, remains in hiding in her old ally India.

"I pray for the eternal peace and forgiveness of Begum Khaleda Zia's soul," Hasina said, in a statement on social media by her now banned Awami League party.

Bangladesh's Prothom Alo newspaper, which said Zia had "earned the epithet of the 'uncompromising leader'", reported that Rahman and other family members were by her side at the time of her death.

"The lives of politicians are marked by rises and falls," the newspaper wrote on Tuesday.

"Lawsuits, arrests, imprisonment, persecution, and attacks by adversaries are far from uncommon. Khaleda Zia endured such ordeals at their most extreme."