From dusk to dawn you must have a white light at the front and a red light at the rear, visible from 300 metres. Steady or flashing is your choice - a flashing lamp is legal in Denmark as long as it blinks at least 120 times a minute. Police enforce this actively, especially in autumn - expect a fine of around 700 kr per missing light.
Rental bikes normally come with lights fitted; check they work before you ride off, and that includes share bikes.
The requirement is a white lamp at the front and a red one at the rear, switched on from dusk to dawn and in poor daytime visibility such as fog or heavy rain. They may shine steadily or flash: flashing lamps are legal in Denmark provided they meet the same colour and visibility requirements and blink at least 120 times a minute. Reflectors are mandatory too, but they do not count as lights.
Either way, a bike shows just one lamp in each direction, and a single lamp makes it hard for anyone to judge how far away you are and how fast you are closing - blinking or not. So treat your lights as the bare minimum rather than a guarantee that you have been placed, and ride accordingly at junctions.
Police run evening checks through the autumn, often near the harbour bridges, and because the fine is charged per missing lamp, two dead lights means two fines. If a rental light fails mid-ride, most shops in our rental guide will swap it for free, and a clip-on lamp from any supermarket or kiosk costs a few kroner and keeps you legal until you get back.