The Civil Human Rights Front said it had told police it expected more than 150,000 people in Tuesday's march, up from 50,000 when it first applied for police permission. The front has increased the number of stewards in the crowd from 150 to 200.
It called on protesters to stay calm and avoid clashes.
The march starts at 3pm at Victoria Park in Causeway Bay and ends at the pedestrian area in Chater Road, Central.
Last year, organisers said 430,000 people marched, although police said 66,000 took part at its peak. An academic at the University of Hong Kong, Professor Paul Yip Siu-fai, counted numbers along the route in Wan Chai and concluded that about 103,000 people marched.
Civil Human Rights Front convenor Johnson Yeung Ching-yin said recent controversies could make a difference this year. They included the Legislative Council's debates on controversial plans to build two new towns in the northeastern New Territories, and Beijing's white paper asserting control over Hong Kong.
Beijing outlined its "comprehensive jurisdiction" over the city in its white paper, which stated that judges were administrators and as such had a "basic political requirement" to love the country. It suggested judges had a responsibility to "correctly understand and implement the Basic Law".
Yeung urged police to close all six lanes outside Victoria Park so the protest flow would be smoother, but police have only promised to close three lanes and the tram lane. "In the past, there have been clashes because they refused to close more lanes for the protesters," he said.
Groups such as the Federation of Students earlier said they might take the initiative and start the Occupy Central civil-disobedience protest after the march.
Yeung said the front would provide back-up, such as legal advice, to the groups but it would not take such action itself.
The march, with a theme of universal suffrage, will be led by villagers who will lose their homes to the development plan in the northeastern New Territories, students and transsexuals.
Meanwhile veteran Beijing loyalist Elsie Leung Oi-sie said it would be a "misunderstanding" to believe that asking judges to be patriotic would damage judicial independence.
Additional reporting by Emily Tsang