President Mohammed Shahabuddin ordered the immediate release of Zia, chief of the main opposition Bangladesh Nationalist Party (BNP), late on Monday after discussing the formation of an interim government with politicians and the army. Hasina fled to India earlier in the day after resigning.

Zia, born on Aug. 15, 1945, has liver disease, diabetes and heart problems, according to her doctors. She has largely remained away from politics for many years.

Popularly known by her first name, Khaleda was described as shy and devoted to raising her two sons until her husband, military leader and then-president of Bangladesh, Ziaur Rahman, was assassinated in an attempted army coup in 1981.

Plunging into politics, she became head of her husband's conservative BNP three years later, vowing to deliver on his aim of "liberating Bangladesh from poverty and economic backwardness".

She joined hands with Hasina, daughter of Bangladesh's founding father and head of the Awami League party, to lead a popular uprising for democracy that toppled military ruler Hossain Mohammad Ershad from power in 1990.

But their cooperation did not last long and the next year, Bangladesh held what has hailed as its first free election with Khaleda winning a surprise victory over Hasina, having gained the support of Islamic political allies.

In doing so, Khaleda became Bangladesh's first female prime minister and only the second woman to lead a democratic government of a mainly Muslim nation after Pakistan's Benazir Bhutto.

Khaleda replaced the presidential system with a parliamentary form of government so that power rested with the prime minister, lifted restrictions on foreign investment and made primary education compulsory and free.

She lost to Hasina in the 1996 elections but came back five years later. But her second term was marred by the rise of Islamist militants and allegations of corruption.

In 2004, a rally that Hasina was addressing was hit by grenades. Hasina survived but over 20 people were killed and more than 500 were wounded. Khaleda's government and its Islamic allies were widely blamed and years later her eldest son was tried in absentia and sentenced to life for the attack. The BNP contended the charges were trumped up.

Although Khaleda later clamped down on Islamist radical groups, her second stint as prime minister ended in 2006 when an army-backed interim government took power amid political instability and street violence.

The interim government jailed both Khaleda and Hasina on charges of corruption and abuse of power for about a year before they were both released ahead of a general election in 2008.

Although the BNP boycotted the 2008 election and Khaleda never regained power, the vitriolic feud with Hasina that led to the two being dubbed "the battling Begums" continued to dominate Bangladeshi politics.

Tension between their two parties has often led to strikes, violence and deaths, impeding economic development for a poverty-stricken country of nearly 170 million that is low-lying and prone to devastating floods.

In 2018, Khaleda, her eldest son and aides were convicted of stealing some $250,000 in foreign donations received by an orphanage trust set up when she was last prime minister - charges that she said were part of a plot to keep her and her family out of politics.

She was jailed but released in March 2020 on humanitarian grounds as her health deteriorated. She has remained under house arrest since then.