Councilor (1967–1975)

Lopez was elected municipal board member (city councilor) of Manila in 1967.

On August 21, 1971, the bombing at Plaza Miranda marred a Liberal Party rally, killing 9 persons and injuring 92 more, including Lopez, who was then a city councilor. President Ferdinand Marcos used the bombing incident as a basis to suspend the writ of habeas corpus.

When Marcos declared martial law, Lopez decided to organize active opposition to the regime that could eventually topple it and help restore democracy to the country. He held secret meetings with labor and student leaders, community and political leaders, exploring ideas on how to organize opposition to the martial law regime.

Laban

Lopez was among the founding signatories of Laban, a contraction of “Lakas ng Bayan” and a Tagalog translation of People's Power. When Marcos announced an election for the Interim Batasang Pambansa on April 7, 1978, he was the campaign manager in Manila, organizing sorties and meetings to attract more citizens to oppose Marcos' dictatorial rule.

Batasang Pambansa (1984–1986)

In recognition of his leadership in Manila, some 20 prospective candidates of United Nationalists Democratic Organizations (UNIDO) for the 1984 Batasang Pambansa elections signed a resolution giving him authority to select the five other candidates for the six assembly seats assigned to Manila based on voting population.

Marcos' ticket, the Kilusang Bagong Lipunan (KBL), suffered a heavy blow and the UNIDO candidates including Lopez and four others won. While serving in the Batasang Pambansa, he filed bills aimed at proposing the compensation income of spouses who are both employees to be taxed separately, paying lower taxes, seeking to abolish the Metro Manila Commission, providing for the establishment of the Philippine National Language Commission, and providing for a synchronization of national and local elections in 1986.

Mayor of Manila (1986–1992)

After the People Power Revolution, Lopez was appointed as Mayor of Manila. During his early years, his administration was faced with ₱700 million pesos worth of debt and inherited an empty treasury. In the first eleven months, however, the debt was reduced to ₱365 million and the city's income rose by around 70%. Revenues from movie theaters rose by 180%, from public markets 170%, and the Manila Zoo began to have profits. Manila had an annual income of over ₱700 million due to these increase in tax revenues. Within the same eleven months, 21 schools were built. Squatters were relocated, with an average of 12,700 families a year.^[4]

During the 1988 elections, he ran for a full term under the PDP-LABAN and won, defeating Lito Atienza and Eva Estrada-Kalaw, both his fellow former delegates at the Batasang Pambansa.

Lopez closed down numerous illegal gambling joints and jueteng. He also blocked the re-opening of jai-alai despite pressure from powerful groups.

In January 1990, Lopez padlocked two Manila casinos operated by the Philippine Amusement and Gaming Corporation (PAGCOR), saying the billions it gained cannot make up for the negative effects gambling inflicts upon the people, particularly the youth. As a result of this, he was charged with graft in the Sandiganbayan in the same year. The case remained unresolved until it was finally dropped in June 2002.^[5]

He also worked for the revival of the Boys’ Town Haven (now referred to as “Boys Town”), rehabilitating its facilities to accommodate underprivileged children and provide them with livelihood and education.

During the 1992 elections, he lost reelection to former National Bureau of Investigation director Alfredo Lim.