The conservative opposition Popular Party is forecast to win the most seats and could govern with the far-right Vox Party, two polls suggested.
Spain's conservative opposition People's Party (PP) was ahead in Sunday's snap election, but short of an outright parliamentary majority, two polls suggested shortly after voting ended on the mainland.
The polls showed that the PP could achieve victory with a potential tie-up with the far-right Vox, a move that has been mooted for months.
One survey by GAD3 for media group Mediaset showed the PP, under leader Alberto Nunez Feijoo, would win 150 seats and Vox 31.
The leftist coalition, led by Prime Minister Pedro Sanchez's Socialists, would get a combined 149 seats in the 350-seat lower chamber, according to GAD3.
Another survey by Sigma Dos pollster for the state broadcaster RTVE also showed the combined rightwing parties winning at the top of the ranges provided (150 for PP and 27 for Vox).
Near-final results are expected by midnight.