UBS Chairman Marcel Ospel, braving investor fury over huge subprime losses, said it was "absolutely necessary" for shareholders to back a 13 billion Swiss franc ($11.94 billion) capital injection from Singapore and an unidentified Middle East investor.
"Today we need your backing for a massive strengthening of our capital base," Mr Ospel told an extrarodinary shareholders' meeting. "We believe that this measure is absolutely necessary."
Mr Ospel has survived at the helm of the giant Swiss bank after culling top managers over the past year as UBS chalked up $18 billion in charges following disastrous investments in US subprime mortgages.
Europe's worst writedowns triggered a loss for 2007, the bank's first since its inception, and forced it to seek 19 billion Swiss francs in emergency capital to repair its depleted capital ratios.
Despite facing calls for his resignation, Mr Ospel said he would not "thoughtlessly relinquish" his responsibilities. "I intend to ensure that UBS gets back on the road to success."
He also said UBS would redouble efforts to cuts it exposures to mortgage-backed securities and derivatives.
The bank had misjudged market trends in subprime and had then failed to react adequately, said Ospel. "We judged certain markets wrongly. We subsequently noticed this error, but due to the rapid evolution of events were unable to react in time."
Earlier, thousands of shareholders converged on a sports arena in Basel for an emergency assembly which started at 9am.
The city's green trams were packed as shareholders, many of them former employees of Switzerland's largest financial institution, made the trip to what is being billed as the most important shareholder gathering in UBS's history.