LILLE, France (AP) _ Protests broke out at French prisons Monday for the third day in a row, and the authorities said steps would be taken to reduce overcrowding and other problems.

In the northern city of Douai, about 50 disgruntled inmates scaled a jailhouse roof Monday and brandished a skull-and-crossbones flag. Police fired tear gas, and the prisoners returned to their cells. A similar rooftop protest had occurred at the prison Sunday.

In Nice, on the French Riviera, about 150 inmates protested overcrowding at their prison by refusing to return to their cells following an afternoon exercise period Monday. They ended the two-hour protest after presenting a list of demands to authorities.

The protests followed two days of rioting by 500 inmates at the Loos-Les- Lille prison in this northern industrial city. Inmates burned a workshop and garage, sacked 175 cells and hurled roof tiles at police, causing an estimated $1.7 million in damage. No serious injuries were reported.

Prison investigators questioned inmates Monday to determine who led the rioting. They promised to uphold negotiators' pledges that no general reprisals would be carried out.

Pierre Arpaillange, the country's top prison official, blamed the riot on overcrowding at Loos-Les-Lilles, which was built for 550 prisoners but holds 1,000. The prison at Nice reportedly holds 900 inmates in a facility built for 280.

Arpaillange told the RTL radio that new prisons for 13,000 inmates are to open by July.

The rebellions were the first in French prisons this year, but there were at least 10 such revolts in 1989. A series of prison revolts in 1987 during an intense heat spell started at the Loos-Les-Lille and Douai facilities.

Loos-Les-Lille is about 125 miles north of Paris and 25 miles from Douai.

Officials said there is no overcrowding at Douai, where 540 inmates are housed in a facility with a capacity for 575.

The demands of the Loos-Les-Lille inmates were never made clear, but officials said they centered on the slowness of the justice system and the parliamentary amnesty recently granted to a number of politicians in a fund- raising scandal.

The Douai prisoners were demanding better recreational facilities.