LONDON, Oct 26 (Reuters) - Shares in some of Europe's biggest banks tumbled in early trading on Thursday after a slew of earnings updates underwhelmed investors, who are already concerned profit margins may have peaked after a long run of central-bank rate rises.

By 0736 GMT, the broader European banking index fell as much as 2.4% to its lowest in four months. Top fallers on the index were Standard Chartered, down 11%, Swedbank, down 7.6% and BNP Paribas, down 5.1%.

London-listed Standard Chartered, which makes the bulk of its profits in Asia, fell by as much as 17% at the open, triggering a brief automatic trading halt.

The FTSE 350 Banks index hit its lowest since March, and was last down 2.5%.

However, several Spanish banks appeared to buck the trend. Sabadell rose around 5% after raising its outlook for 2023 net interest income growth on the back of higher interest rates. (Reporting by Iain Withers and Naomi Rovnick in London, Jesus Aguado in Madrid and Danilo Masoni in Milan; Editing by Amanda Cooper)