Alistair Darling has warned that an independent Scotland would lack diplomatic clout in dealing with international crises like Syria.
Speaking at the launch of the Glasgow Better Together campaign, Mr Darling said the UK has "far more clout... than a smaller country ever would."
Leaders from the Better Together camp met today for the official Glasgow launch of their campaign, setting out the case for Scotland remaining in the UK with the message it would be the "best of both worlds".
The former chancellor was joined by the three pro-UK Scottish party leaders - Labour's Johann Lamont, Lib Dem Willie Rennie and Ruth Davidson of the Conservatives - at the city's Mitchell Theatre for a speeches and a Q&A session.
Responding to a question about the controversial Commons vote against potential military action in Syria, Mr Darling, who leads the Better Together campaign, said: "Unfortunately and tragically, the atrocities that are being visited upon the people of Syria are going to continue."
"Ultimately, the long term solution is going to have to be diplomatic and the UK has got far more clout in the United Nations, in the G20 which is meeting in Russia next week, than a smaller country ever would.
"I've been to lots of these meetings throughout the time I was a minister and large countries, particularly one with a reputation like ours, have clout.
"I don't think there's anyone in this room who wouldn't want to see the clout of the UK, if possible, along with other countries, trying to bring an end to the bloodshed taking place in Syria."
The event was met by a handful of pro-Yes protesters who had gathered outside the theatre. One audience member was asked to leave after repeatedly heckling the panel during the Q&A session. Initially directing a question to Ruth Davidson, he attacked the presence of Trident nuclear weapons in Scotland - to boos from the audience - before being escorted from the theatre.
The 400-seat theatre was almost full, and among those present were Glasgow City Council leader Gordon Matheson, Labour MSPs Drew Smith and Hanzala Malik, and MPs Tom Harris and Ann McKechin.
Johann Lamont opened her speech by saying Alex Salmond was putting the referendum ahead of the interests of the people of Scotland. She said: "Scotland deserves better than what we have at the moment. We deserve better living standards, better schools and hospitals, secure jobs and a vision of what Scotland can be.
"But at the moment we cannot address the pressing needs of you and me, of families throughout our country because Alex Salmond has the nation on pause as we await his referendum."
Later, Scottish Liberal Democrat leader Willie Rennie described staying in the UK as "the best of both worlds". He said: "We are stronger when we work together. We have the best of both worlds: we have a strong Scottish Parliament making real decisions here in Scotland about Scotland."
Mr Darling said that those who favoured Scotland staying in the union could not afford to be complacent, but warned against the "gimmicky" nature of Alex Salmond's "indy-lite".
He said: "Indy-lite is exactly what it sounds like. A gimmick cooked up by advertising men to get us to buy a product we don't need.
"Every day for the next year we will make our case that a successful Scottish Parliament backed-up by the strength of a bigger United Kingdom offers us the best of both worlds. Increasingly the independence debate is becoming a battle between those of us offering to continue the success of devolution versus those selling the dishonesty of indy-lite."
Ruth Davidson focused on the armed forces, the NHS and the opportunities available for young people in Scotland, and stressed the role of the UK played in building these.
She said: "I see that at the moment we do have the best of both worlds, we do have control over health, over education, over policing, through the Scottish Parliament. But we also have strength from standing with our friends, standing with our family members in other parts of the UK and standing with the other four nations."
The Yes campaign dismissed the idea that staying in the UK was good for Scotland.
A spokesman for Yes Scotland said: "We certainly agree with Johann Lamont that Scotland deserves better than we have at the moment. But why would anybody believe that sticking with a failed Westminster system that is responsible for imposing policies such as the punitive bedroom tax is the way to achieve that?"
He added: "One of the key advantages of becoming independent is to foster new, healthy and equal relationships with other countries in these isles. Independence for Scotland is also good for the rest of the UK.
"With a Yes vote, we will be taking control of our own future and making decisions that match our own priorities and aspirations. And, unlike now under the Westminster system, we will always get the government we vote for."
The spokesman also claimed the launch of Better Together's Glasgow campaign came "nine months after the hugely-successful launch of Yes Glasgow which was attended by more than 700 people". He added: "Yes Glasgow has staged nearly 240 events, has more than a thousand volunteers across the city and some 500 Yes ambassadors."