Mr Capriles is seeking support for contesting his narrow loss in April's Venezuelan presidential election.

Venezuela's foreign minister said the meeting would "derail" good relations between the two countries.

His Colombian counterpart said she did not want to debate the issue through the media.

Venezuelan parliament speaker Diosdado Cabello compared the meeting to "placing a bomb on the train".

Mr Capriles said he did not care "what this illegitimate government says". He compared the criticism of his meeting with President Santos to "mosquito bites".

Venezuela is threatening to stop acting as a facilitator in peace talks between the Colombian government and Colombia's biggest rebel group, the Revolutionary Armed Forces of Colombia (Farc) because of the meeting.

"Without a doubt today's situation obliges us to study Venezuela's participation as a facilitator in the peace accord,'' Foreign Minister Elias Jaua said.

He said that Venezuela had recalled its delegate to the peace talks, arguing that it was "very difficult to work toward peace for a brother nation when the major institutions of that nation are provoking and stimulating the destabilization of Venezuela".

The talks are currently in recess after both sides agreed a deal on agrarian reform. They are set to resume on 11 June.

Mr Capriles alleges that Venezuelan President Nicolas Maduro "stole" April's election through fraud and voter intimidation.

Venezuela's electoral council is currently conducting an audit of the votes, which Mr Capriles has dismissed as a "farce".

President Santos has recognized Mr Maduro's election victory and attended his inauguration last month.

Mr Maduro won by a margin of 1.49 percentage points over Mr Capriles, according to official figures released by the electoral council.