KARACHI, PAKISTAN, JULY 5 -- Seven persons, including two children, died and 50 were injured when three bombs went off at a crowded railway station and a bus terminal in the eastern Pakistani city of Lahore today, police officials said.

The bombing came on the eve of a massive gathering of Pakistani Shiites in Lahore to launch a pro-Iranian political party.

Officials investigating the bombings said they may have been intended to heighten tension between Shiite and Sunni Moslems. They pointed specifically to the Afghan secret service, Khad.

Meanwhile, protest rallies were held in most Pakistani cities to mark what the opposition calls the "black day," the 10th anniversary of the coup overthrowing Zulfiqar Ali Bhutto that brought Gen. Mohammed Zia ul-Haq to power.

The bombings in Lahore followed one in May that killed six persons, including the head of an extremist pro-Saudi Arabia Sunni group during its annual public meeting. A major faction in the party accused the government of causing the blast, but another strong faction of the party blamed pro-Iranian Shiites.

Intelligence officers in Karachi say they believe that hard-core Shiite and Sunni organizations receive financial and ideological aid from Iran, Iraq and Saudi Arabia.

Troops were called last year to quell violent clashes between Shiites and Sunnis in Lahore.

Despite today's bombing threats from Sunni organizations, Shiite leaders in Lahore said they were determined to hold their public meeting on Monday at an unspecified place.

The "black day" protest rallies were called by the 11-party Movement for the Restoration of Democracy.

Hundreds of activists of the Pakistan People's Party, headed by Bhutto's daughter Benazir, marched through Karachi demanding Zia's resignation.

The 10th anniversary of Zia's rule comes at a time when the political climate of the country, which was boiling after Benazir Bhutto's return to the country last year, had cooled. Bhutto has stopped holding big public meetings and leading processions. She has spent the last six months reorganizing the People's Party to prepare for elections expected not earlier than 1990.

Unlike last year, when she had made two public appearances on the ninth anniversary of the coup, she spent most of the day today at her house, where she saw off her party's procession. It later marched to the Nazimabad district, where opposition leaders addressed a rally of about 20,000.

In a major concession to the opposition, Prime Minister Mohammed Khan Junejo allowed all protest rallies today. Police were also ordered to keep a low profile and not to make any arrests of political activists, unlike last year.