BUENOS AIRES--Argentina's black-market peso weakened sharply Thursday just hours after Congress passed laws that analysts say will weaken the judicial branch as well as the ability of people and companies to protect their private property rights.

On the black market, the peso weakened to ARS9.20 against the U.S. dollar from around ARS8.94 in the previous session, according to the newspaper El Cronista, which publishes an average of rates collected from black-market traders.

Argentina's official peso was barely changed at ARS5.174 to the dollar on the regulated MAE foreign-exchange wholesale market, compared with ARS5.175 previously.

Argentine stocks rose, with the Merval index gaining 2.28% to close at 3,685.62.

Steel producer Siderar SAIC (ERAR.BA) led the gains, rising 6.4% to ARS2.34. Telecom Argentina SA (TECO2.BA, TEO) was the most traded stock. It rose 3% to ARS29.10. The steel producer Tenaris SA (TEN.MI, TS) rose almost 2% to ARS193.70.

The price of the TVPY GDP warrant, whose performance is tied to economic growth, rose 1.52% to ARS50.25. The Boden 2015 bond rose 1.32% in price terms to ARS788.

Write to Taos Turner at taos.turner@dowjones.com