* Election poll puts Catalan secessionists behind
* Italy receives unexpected rating upgrade from S&P
* Portuguese bond yields lowest in 2-1/2 years (Adds quote, updates prices)
LONDON, Oct 30 (Reuters) - Spanish and Italian government borrowing costs fell on Monday after an opinion poll suggested separatists in Spain's Catalonia region may lose a December election, and Rome received its first rating upgrade from Standard & Poor's in at least three decades.
The poll, which showed a small lead for parties opposing a split, was the first since Madrid called the Dec. 21 vote last Friday after dismissing Catalonia's regional government and dissolving its parliament.
On Sunday, hundreds of thousands of supporters of a unified Spain filled Barcelona's streets in one of the biggest shows of force by those who have largely looked on as regional leaders pushed for independence.
Italy on Friday received an unexpected upgrade in its debt rating from BBB- to BBB, with S&P citing a strengthening economic outlook, growing investment, a steady rise in employment and improvements in the debt-laden banking sector.
Investors welcomed the developments. Spain's and Italy's 10-year bond yields - which move inversely to price - fell 4-7 basis points on Monday, with the latter hitting its lowest level in over seven weeks.
Portuguese bonds, another so-called peripheral market that tends to trade in line with Spanish and Italian peers, were also caught up in the rally, with 10-year yields down 6 bps at their lowest level in around 2-1/2 years.
"All arguments are in favour of peripheral debt. Portugal is in the tailwind of Spain and Italy," said Commerzbank's head of rates research, Christoph Rieger.
Spanish bonds were the best performer on the day, with 10-year yields down as much as 7 bps at 1.51 percent , recovering some of the ground lost in recent weeks as the political crisis in Catalonia deepened.
A 4 bps fall in their Italian equivalents to 1.91 percent helped hold Italy's borrowing costs close to their lowest level this year relative to Germany, the bloc's benchmark borrower .
The European Central Bank's decision last week to extend its asset purchase programme to at least September 2018, albeit at a slower pace, has also encouraged investors back into riskier debt and stock markets.
Spain's stock market gained 1.3 percent on Monday, while Italy's main index was up 0.4 percent.
The euro was up 0.2 percent against the dollar at $1.1633 , recovering from a three-month low struck on Friday. (Reporting by John Geddie and Fanny Potkin; Editing by Kevin Liffey)