"Please help me, please help me," Hassan, who works for aid agency Care International, was shown saying while crying. "This might be my last hour."
"Please, the British people, ask Mr Blair to take the troops out of Iraq and not to bring them here to Baghdad. That's why people like Mr Bigley and myself are being caught," she said, referring to British captive Kenneth Bigley, who was decapitated by his captors earlier this month.
"Maybe I will die like Mr Bigley," Irish-born Hassan said, before burying her head in her hands in tears.
Hassan, has lived in Iraq for 30 years and holds Iraqi, Irish and British citizenship. She was kidnapped in Baghdad on Tuesday.
Blair under pressure
Hassan's plea came one day after Britain agreed to a US request to redeploy 850 troops to Babil province south and south-west of Baghdad, freeing up US soldiers for an expected assault on Falluja.
Nasser al-Badri, Aljazeera's correspondent in London, said the tape would put increasing pressure on British Prime Minister Tony Blair.
Hassan asked for the withdrawal
of British troops
"Pressure is mounting against Blair, not only from opposition parties, including the Liberal Democrats, but also from his own Labour party whose rank and file have loudly rejected the government's policy in Iraq, and especially the recent deployment of about 850 British troops near Baghdad as requested by the US," he said.
"Certainly, the tape of Margaret Hassan would have profound impact on the British public opinion, which is largely against the government's policy in Iraq and wants to see an early return of British troops from Iraq."
Foreign Office comment
Blair's office declined to immediately comment on Hassan's plea. "We have no comment," a Downing Street spokesman said.
But the UK Foreign and Commonwealth Office released a statement from Jack Straw.
"I have the greatest sympathy for what her family is suffering," Straw said.
"The one thing you do in hostage negotiations is to keep absolutely quiet and walk on eggshells. Britain and America are the most hated people in the Middle East, while Ireland is highly regarded. You don't just jump into a situation like this which is so delicate"
friend of Margaret Hassan
"Margaret Hassan has spent more than 30 years working for the Iraqi people. We hope all Iraqis will join us in calling for her immediate release."
But the statement made no reference to any negotiations or steps London might take to help secure her release.
In Bigley's case, Britain said it would not negotiate over his captors' demands, but did manage to contact them through an intermediary days before they killed him.
Irish Prime Minister Bertie Ahern has called for the immediate release of Hassan.
In a statement received by Aljazeera, Ahern said: "The Irish government and people have been shocked by the abduction of Mrs Margaret, who was born in Ireland and still holds Irish nationality.
"Margaret has dedicated her life and worked for the interests of the Iraqis and struggled throughout her life to protect the rights of the Iraqi people and, as such, her abduction amidst her family was a harsh and shocking act," he said.
On Friday, a friend of Hassan's, Felicity Arbuthnot, sharply criticised Blair, claiming on BBC radio that comments he had made about the kidnappers could reduce her chances of being freed.
Blair comments criticised
On Tuesday, following the broadcast of the first video of Hassan in captivity, Blair said: "This is someone who has lived in Iraq for 30 years, someone who is immensely respected, someone who is doing their level best to help the country. I think it shows you the type of people we are up against. We will do whatever we can, obviously."
The following day, Blair made similar comments in the House of Commons.
Arbuthnot was quoted by the BBC as saying that Blair had effectively linked Hassan in the kidnappers' minds with the British government and made it less likely that she would be able to take advantage of her birthplace in Dublin or her Iraqi nationality to help secure her freedom.
Arbuthnot said by criticising the kidnappers, Blair could have angered them, placing Hassan's life at greater risk, given the key role that British forces have played in the Iraq war.
"The one thing you do in hostage negotiations is to keep absolutely quiet and walk on eggshells," she said.
"Britain and America are the most hated people in the Middle East, while Ireland is highly regarded. You don't just jump into a situation like this which is so delicate."