They will turn on, likely with slightly uneven brightness - the LEDs internal resistance and the pins limited current drive will act like the current limiter.
The actual current will depend on the supply voltage, led used and the mcus drive capabilities.
For a red 4 digit 7 seg on an atmeha8, I got less than 10ma at 2,5v and 20ma at 3.3v, with the LEDs multiplexed.
Edit: some quick math.
Say that the current through each led segment is I. And the gpio resistance is R.
Each common digit has 8 I current through it, and each segment has 4 I. Some math later,
I = (Vcc - Vf) / (12 R) where Vcc is the supply voltage and Vf is the LEDs forward voltage (1.9v for a typical red led). The gpio resistance varies from 50+ ohm to 30+ ohm, higher at lower voltage.
Plus in 3.3v and 2.7v you get total current of 30 to 75ma for all digits.
I'm going to measure mine later.
edit: about 25ma (total current = mcu + led) at 2.5v and 50ma at 3.3v. the mcu consumes 2-3ma approximately.
the brightness is fairly even amongst the leds if you don't look too closely.