Governments raise money by the sale of interest-bearing bonds (called 'gilts')
http://www.dmo.gov.uk...age=gilts/about_gilts
Since gilts are tradeable assets, they can end up in the hands of anyone. In practice that means that they usually end up in the hands of those governments which have a financial surplus (or with the banks of those countries, which are often state-controlled anyway).
As Zita-green indicates. the country which currently has the most 'financial clout' is China. The Chinese could bankrupt the USA overnight if they exercised their option to call in the debt owed on US bonds. (They would never do so because the resulting financial chaos would harm the Chinese economy just as much as that of the USA).
The system works well as long as countries can be trusted to repay their debts. Greece was close to the point of national bankruptcy (which could have resulted in the type of chaos experienced in Zimbabwe) but has the support of other Euro-zone countries to call upon. To a lesser extent, Spain, Portugal and Ireland are in a similar position. The UK has broadly similar problems to those countries but, unlike the other countries mentioned, it can't call upon fellow Euro-zone countries for assistance.
Chris