The strikes are the latest in a series of walkouts which have previously also involved staff in Ireland and Sweden. "We are lemons to be squeezed and thrown away", said Juan Fernandez, a former Ryanair employee who joined protesters at Brussels airport.
Ryanair said the majority of its 2,400 flights today would not be hit by the new strikes, with roughly 35,000 of 450,000 passengers experiencing disruption.
"Today, over 2,150 Ryanair flights (90 percent of our schedule) will operate as normal carrying 400,000 customers across Europe", the airline said in a statement.
Strikers at the airport held up placards with the message "RYANAIR MUST CHANGE".
Thousands of travellers in Europe saw their plans disrupted Friday after airline Ryanair cancelled more than 200 flights because of strikes by cabin crew in Spain, Belgium, Holland, Portugal, Italy and Germany.
Friday's strike will be the second biggest one-day strike after some 55,000 customers were put out in August when pilots in five European countries walked out during the peak of the summer holiday season.
Shares in Ryanair were 0.2% higher at 13.18 euros by 1240 GMT, having fallen 20% since the action ramped up in mid-July.
The low-priced rival, which is in positive territory in market value terms, reported on Friday that it was expecting annual profits to come in at the higher end of expectations after bookings benefited from Ryanair's series of cancellations.
In a statement Thursday, the company blamed cabin crews of competitors for organizing the strikes and blocking "significant progress" in labour negotiations.
The dispute in other markets chiefly centres on Ryanair's practice of employing a large proportion of its staff under Irish law, which unions say inconveniences staff and impedes them from accessing local social security benefits.
Ryanair this week signed deals with cabin crew unions in Italy to provide employment contracts under Italian law and agreed to arbitration with the union representing its German pilots.
The airline, which has been battling labour unrest for nearly a year, has blasted the decision of German pilot union Vereinigung Cockpit (VC) to call the strike there.
The German pilots' union said it could not rule out further strikes. "We want the company to change", union representatives Ingolf Schumacher told Reuters.