(Adds finance minister comment)
By Julia Symmes Cobb and Nelson Bocanegra
BOGOTA, Jan 30 (Reuters) - Colombia's central bank held its key lending rate for a fifth straight month on Friday, in an effort to boost slowing economic growth as revenue from oil price declines and mounting problems in Europe increase uncertainty.
The bank's seven-member board voted unanimously to keep the rate at 4.5 percent, in line with the forecast of analysts polled by Reuters this week.
The bank lowered its 2015 economic growth forecast to a "most-probable" 3.6 percent from 4.3 percent previously.
"This decline reflects growth in domestic demand that is adjusting itself to lower levels of national income," the central bank said in a statement. "The sharp drop in oil prices is already reflected in cuts in investment programs in the sector."
Even so, Finance Minister Mauricio Cardenas said portfolio investment was flowing into the economy, offsetting some of the lost investment as foreign players remain undeterred. Oil is Colombia's biggest export.
"There has been a very important entrance of portfolio investment, which shows that despite the fall in oil prices, confidence remains intact and foreigners are still bringing dollars into the country at an unprecedented rate," said Cardenas, who represents the government on the board.
The bank also trimmed its forecast for full-year 2014 growth to a likely 4.8 percent, down from 5 percent. The economy accelerated 4 percent in the fourth quarter, the bank estimated.
The reduction in growth projections opens the door to interest rate cuts this year, analysts said. Fourth quarter data is due in March.
"We think that in the short term the bank will keep the rate steady," Bancolombia said in a note to investors.
"However, if inflation loses strength and the expansion of internal demand is affected by external risks, we think that by the end of the first half of the year there will be space for a cut to the rate."
Borrowing costs were raised 125 basis points last year after faster-than-expected first-quarter growth raised concerns about consumer prices.
Inflation for 2015 will be slightly above 3 percent, the bank said, lower than the 3.66 percent in 2014.
The board did not take any measures to curb volatility of the peso, which weakened around 12 percent in December alone and by around a fifth over a year.
The board said the adverse effects of the drop in oil prices would be partly offset by the peso weakening, which will boost the competitiveness of exports. (Additional reporting by Helen Murphy, Peter Murphy, Luis Jaime Acosta and Carlos Vargas; Editing by Meredith Mazzilli and Chizu Nomiyama)