Heavy action on Juan Manuel Marquez hours before fourth fight with Manny Pacquiao

The line on Juan Manuel Marquez, who opened as a 3-to-1 underdog, have moved closer to even odds the fight draws near. (Jeff Bottari/Getty Images)

LAS VEGAS -- The shifting odds at the MGM Grand sports book hours before Saturday's fourth fight between Manny Pacquiao and Juan Manuel Marquez are reflecting a rush of action on the underdog.

Marquez started as a +300 longshot when betting opened on Sept. 19, meaning a $100 wager returns $300 in winnings. Those odds had been slashed to +260 by Friday afternoon, after the bulk of Marquez fans arrived and created a vocal presence at the weigh-ins.

Yet the line on a Marquez victory was down to +175 as of 1:30 p.m. local time on Saturday, approximately seven hours before the non-title fight (9 pm. ET, HBO PPV, $59.99).

Pacquiao, who opened as a -400 favorite (meaning a $400 bet returns $100 in winnings), had fallen to -370 on Friday. Those odds had been slashed to -220 by Saturday afternoon.

• Experts' predictions on Pacquiao-Marquez IV

Marquez was a far more lopsided underdog -- opening at +500 -- for his third fight with Pacquiao in November 2011.

That fight ended in a controversial majority decision for the Filipino, with a majority of the media at ringside scoring it a draw or a narrow victory for the Mexican. The announcement of that outcome by Michael Buffer prompted a cascade of boos from the heavily pro-Marquez crowd, many of whom hurled bottles and cans toward the ring.

-- Bryan Armen Graham