COPENHAGEN, Denmark (AP) _ Denmark took an unsurmountable 3-0 lead over Belgium Saturday as Peter Bastiansen and Michael Mortensen downed Alain Brichant and Jan van Langendonck 6-4, 3-6, 6-2, 6-3 in the doubles of their Davis Cup European Zone A second- round match.

Earlier in the day, Mortensen finished up his singles victory over van Langendonck, defeating the Belgium 57, 1-6, 6-1, 6-1 6-3. The match had been halted by darkness Friday night after the two had split the first four sets and they were tied 2-2 in the fifth.

In the doubles, the Belgians mounted a threat only in the second set, where the Danes led 3-1 but lost their concentration with Bastiansen netting six straight easy balls. Brichant played extremely well at that stage, but then faded, and the Danes took firm command.

Denmark is trying to win back the berth in the Davis Cup World Group it lost to India last year. ---

ISTANBUL, Turkey (AP) - Florin Segerceanu and Dirzu Andrei defeated Turkey's Yavuz Erkangil and Muzaffer Arpacioglu 6-4, 6-3, 6-2 Saturday to give Romania an unbeatable 3-0 lead in the European Zone B Davis Cup second round.

The ''reverse'' singles will be played Sunday. ---

ATHENS, Greece (AP) - Austria took an unbeatable 3-0 lead over Greece in a second round European Zone B Davis Cup match after winning the doubles on Saturday.

Alexander Antonitsch and Peter Fiegl downed Greece's Constantine Efraimoglou and George Kalovelonis 8-6, 6-3, 7-5.

The two remaining singles will be played Sunday. ---

NOTTINGHAM, England (AP) - Britain took a winning 3-0 lead in a European Zone B second-round Davis Cup match against Portugal Saturday when Stephen Shaw and Jeremy Bates downed Joao Silva and Pedro Cordeiro 6-3, 6-4, 6-2 in the doubles.

The two British players, who both won their singles on Friday, breezed to victory in 79 minutes without losing their serves.

The Portuguese pair, used to playing on slow clay courts, were unable to adapt their styles to the fast grass surface.

Cordeiro, who is on leave from the Portguese army, was the weak link as he was the only player to drop his serve in the opening two sets. His partner served well and also produced some fine volleys, but the British pair never were put under pressure. ---

WEGGIS, Switzerland (AP) - Switzerland on Saturday clinched victory in its European Zone B Davis Cup tennis match against Zimbabwe by taking a 3-0 lead after the doubles.

Heinz Guenthardt teamed with Jakob Hlasek in trouncing Haroon Ismail and Philip Tuckniss 6-1, 6-1, 6-1. ---

MONTE CARLO, Monaco (AP) - Ireland held a 2-1 lead over Monaco in a second- round European Zone A Davis Cup tennis match, despite losing in the doubles on Saturday.

Bernard Balleret and Jacques Vinceloni of Monaco defeated Matt Doyle and Sean Sorensen of Ireland 6-3, 2-6, 4-6, 6-4, 6-4.

On Friday, Doyle won a five-set match, downing Gilles Ganancia 5-7, 4-6, 6-4, 6-0, 6-4, and Sorensen triumphed over Balleret 5-7, 6-2, 7-5, 6-1.

Ireland can win the match with a victory in one of the two ''reverse'' singles on Sunday.